selftests: udp: test for passing SO_MARK as cmsg

Before fix:
|  Case IPv6 rejection returned 0, expected 1
|FAIL - 1/4 cases failed

With the fix:
| OK

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>