selftests/bpf: fix sendmsg6_prog on s390

"sendmsg6: rewrite IP & port (C)" fails on s390, because the code in
sendmsg_v6_prog() assumes that (ctx->user_ip6[0] & 0xFFFF) refers to
leading IPv6 address digits, which is not the case on big-endian
machines.

Since checking bitwise operations doesn't seem to be the point of the
test, replace two short comparisons with a single int comparison.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>