[NETFILTER]: nfnetlink_log: micro-optimization: inst->skb != NULL in __nfulnl_send()

No other function calls __nfulnl_send() with inst->skb == NULL than
nfulnl_timer().

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>