packet: propagate sock_cmsg_send() error

sock_cmsg_send() can return different error codes and not only
-EINVAL, and we should properly propagate them.

Fixes: c14ac945

 ("sock: enable timestamping using control messages")
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>