[NETFILTER]: nfnetlink_log: fix use after free

Paranoia: instance_put() might have freed the inst pointer when we
spin_unlock_bh().

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>