[NETFILTER]: nfnetlink_log: micro-optimization: don't modify destroyed instance

Simple micro-optimization: Don't change any options if the instance is
being destroyed.

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>