[NETFILTER]: nfnetlink_log: fix instance_create() failure path

Fix memory leak on instance_create() while module is being unloaded.

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>