[NETFILTER]: nfnetlink_log: fix module reference counting

Count module references correctly: after instance_destroy() there
might be timer pending and holding a reference for this netlink instance.

Based on patch 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>