tc_filter: fix unitialized warning

tc_filter: fix unitialized warning

When run with -fanalyzer.

tc_filter.c: In function ‘tc_filter_list’:
tc_filter.c:718:17: warning: use of uninitialized value ‘chain_index’ [CWE-457] [-Wanalyzer-use-of-uninitialized-value]
  718 |                 addattr32(&req.n, sizeof(req), TCA_CHAIN, chain_index);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ‘do_chain’: events 1-4
    |
    |  772 | int do_chain(int argc, char **argv)
    |      |     ^~~~~~~~
    |      |     |
    |      |     (1) entry to ‘do_chain’
    |  773 | {
    |  774 |         if (argc < 1)
    |      |            ~
    |      |            |
    |      |            (2) following ‘true’ branch (when ‘argc <= 0’)...
    |  775 |                 return tc_filter_list(RTM_GETCHAIN, 0, NULL);
    |      |                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |                        |
    |      |                        (3) ...to here
    |      |                        (4) calling ‘tc_filter_list’ from ‘do_chain’
    |
    +--> ‘tc_filter_list’: events 5-8
           |
           |  582 | static int tc_filter_list(int cmd, int argc, char **argv)
           |      |            ^~~~~~~~~~~~~~
           |      |            |
           |      |            (5) entry to ‘tc_filter_list’
           |......
           |  597 |         __u32 chain_index;
           |      |               ~~~~~~~~~~~
           |      |               |
           |      |               (6) region created on stack here
           |      |               (7) capacity: 4 bytes
           |......
           |  601 |         while (argc > 0) {
           |      |                ~~~~~~~~
           |      |                     |
           |      |                     (8) following ‘false’ branch (when ‘argc <= 0’)...
           |
         ‘tc_filter_list’: event 9
           |
           |../include/uapi/linux/pkt_sched.h:72:35:
           |   72 | #define TC_H_MAKE(maj,min) (((maj)&TC_H_MAJ_MASK)|((min)&TC_H_MIN_MASK))
           |      |                             ~~~~~~^~~~~~~~~~~~~~~
           |      |                                   |
           |      |                                   (9) ...to here
tc_filter.c:698:26: note: in expansion of macro ‘TC_H_MAKE’
           |  698 |         req.t.tcm_info = TC_H_MAKE(prio<<16, protocol);
           |      |                          ^~~~~~~~~
           |
         ‘tc_filter_list’: events 10-16
           |
           |  702 |         if (d[0]) {
           |      |            ^
           |      |            |
           |      |            (10) following ‘false’ branch...
           |......
           |  707 |         } else if (block_index) {
           |      |                   ~~~~~~~~~~~~
           |      |                   ||
           |      |                   |(11) ...to here
           |      |                   (12) following ‘false’ branch...
           |......
           |  717 |         if (filter_chain_index_set)
           |      |            ~~~~~~~~~~~~~~~~~~~~~~~
           |      |            ||
           |      |            |(13) ...to here
           |      |            (14) following ‘true’ branch...
           |  718 |                 addattr32(&req.n, sizeof(req), TCA_CHAIN, chain_index);
           |      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           |      |                 |
           |      |                 (15) ...to here
           |      |                 (16) use of uninitialized value ‘chain_index’ here
           |
tc_filter.c:718:17: warning: use of uninitialized value ‘chain_index’ [CWE-457] [-Wanalyzer-use-of-uninitialized-value]
  718 |                 addattr32(&req.n, sizeof(req), TCA_CHAIN, chain_index);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ‘do_filter’: events 1-4
    |
    |  744 | int do_filter(int argc, char **argv)
    |      |     ^~~~~~~~~
    |      |     |
    |      |     (1) entry to ‘do_filter’
    |  745 | {
    |  746 |         if (argc < 1)
    |      |            ~
    |      |            |
    |      |            (2) following ‘true’ branch (when ‘argc <= 0’)...
    |  747 |                 return tc_filter_list(RTM_GETTFILTER, 0, NULL);
    |      |                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |                        |
    |      |                        (3) ...to here
    |      |                        (4) calling ‘tc_filter_list’ from ‘do_filter’
    |
    +--> ‘tc_filter_list’: events 5-8
           |
           |  582 | static int tc_filter_list(int cmd, int argc, char **argv)
           |      |            ^~~~~~~~~~~~~~
           |      |            |
           |      |            (5) entry to ‘tc_filter_list’
           |......
           |  597 |         __u32 chain_index;
           |      |               ~~~~~~~~~~~
           |      |               |
           |      |               (6) region created on stack here
           |      |               (7) capacity: 4 bytes
           |......
           |  601 |         while (argc > 0) {
           |      |                ~~~~~~~~
           |      |                     |
           |      |                     (8) following ‘false’ branch (when ‘argc <= 0’)...
           |
         ‘tc_filter_list’: event 9
           |
           |../include/uapi/linux/pkt_sched.h:72:35:
           |   72 | #define TC_H_MAKE(maj,min) (((maj)&TC_H_MAJ_MASK)|((min)&TC_H_MIN_MASK))
           |      |                             ~~~~~~^~~~~~~~~~~~~~~
           |      |                                   |
           |      |                                   (9) ...to here
tc_filter.c:698:26: note: in expansion of macro ‘TC_H_MAKE’
           |  698 |         req.t.tcm_info = TC_H_MAKE(prio<<16, protocol);
           |      |                          ^~~~~~~~~
           |
         ‘tc_filter_list’: events 10-16
           |
           |  702 |         if (d[0]) {
           |      |            ^
           |      |            |
           |      |            (10) following ‘false’ branch...
           |......
           |  707 |         } else if (block_index) {
           |      |                   ~~~~~~~~~~~~
           |      |                   ||
           |      |                   |(11) ...to here
           |      |                   (12) following ‘false’ branch...
           |......
           |  717 |         if (filter_chain_index_set)
           |      |            ~~~~~~~~~~~~~~~~~~~~~~~
           |      |            ||
           |      |            |(13) ...to here
           |      |            (14) following ‘true’ branch...
           |  718 |                 addattr32(&req.n, sizeof(req), TCA_CHAIN, chain_index);
           |      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           |      |                 |
           |      |                 (15) ...to here
           |      |                 (16) use of uninitialized value ‘chain_index’ here
           |

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>