netfilter: xtables: use memcmp in unconditional check

Instead of inspecting each u32/char open-coded, clean up and make use
of memcmp. On some arches, memcmp is implemented as assembly or GCC's
__builtin_memcmp which can possibly take advantages of known
alignment.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>