netfilter: ctnetlink: fix echo if not subscribed to any multicast group

This patch fixes echoing if the socket that has sent the request to
create/update/delete an entry is not subscribed to any multicast
group. With the current code, ctnetlink would not send the echo
message via unicast as nfnetlink_send() would be skip.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>