[NETFILTER]: ctnetlink: check for listeners before sending expectation events

This patch uses nfnetlink_has_listeners to check for listeners in
userspace.

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>