iptables: Properly clear iptables_command_state object

iptables: Properly clear iptables_command_state object

When adding a rule with a target which defines a udata_size, valgrind
prints:

8 bytes in 1 blocks are definitely lost in loss record 1 of 1
   at 0x484659F: calloc (vg_replace_malloc.c:1328)
   by 0x486B128: xtables_calloc (xtables.c:434)
   by 0x1128B4: xs_init_target (xshared.c:238)
   by 0x113CD3: command_jump (xshared.c:877)
   by 0x114969: do_parse (xshared.c:1644)
   by 0x10EEB9: do_command4 (iptables.c:691)
   by 0x10E45B: iptables_main (iptables-standalone.c:59)
   by 0x49A2349: (below main) (in /lib64/libc.so.6)

It is not sufficient to free cs.target->t, so call
xtables_clear_iptables_command_state() which takes care of all the
details.

Fixes: 2dba676b68ef8 ("extensions: support for per-extension instance "global" variable space")
Signed-off-by: Phil Sutter <phil@nwl.cc>