[NETFILTER]: nfnetlink_log: flush queue early

If queue is filled to its threshold, then flush it right away instead
of waiting for timer or next packet.

Signed-off-by: Michal Miroslaw <mirq-linux@rere.qmqm.pl>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>