[NETFILTER]: nfnetlink_log: kill duplicate code

Kill some cut'n'paste effect.
Just after __nfulnl_send() returning, inst->skb is always NULL.

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>