netlink_linearize: avoid strict-overflow warning in netlink_gen_bitwise()

netlink_linearize: avoid strict-overflow warning in netlink_gen_bitwise()

With gcc-13.2.1-1.fc38.x86_64:

  $ gcc -Iinclude -c -o tmp.o src/netlink_linearize.c -Werror -Wstrict-overflow=5 -O3
  src/netlink_linearize.c: In function ‘netlink_gen_bitwise’:
  src/netlink_linearize.c:1790:1: error: assuming signed overflow does not occur when changing X +- C1 cmp C2 to X cmp C2 -+ C1 [-Werror=strict-overflow]
   1790 | }
        | ^
  cc1: all warnings being treated as errors

It also makes more sense this way, where "n" is the hight of the
"binops" stack, and we check for a non-empty stack with "n > 0" and pop
the last element with "binops[--n]".

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>