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>

tools/testing/selftests/bpf/progs/sendmsg6_prog.c

@@ -41,8 +41,7 @@ int sendmsg_v6_prog(struct bpf_sock_addr *ctx)
 	}
 
 	/* Rewrite destination. */
-	if ((ctx->user_ip6[0] & 0xFFFF) == bpf_htons(0xFACE) &&
-	     ctx->user_ip6[0] >> 16 == bpf_htons(0xB00C)) {
+	if (ctx->user_ip6[0] == bpf_htonl(0xFACEB00C)) {
 		ctx->user_ip6[0] = bpf_htonl(DST_REWRITE_IP6_0);
 		ctx->user_ip6[1] = bpf_htonl(DST_REWRITE_IP6_1);
 		ctx->user_ip6[2] = bpf_htonl(DST_REWRITE_IP6_2);