[NETFILTER]: nfnetlink_log: don't count max(a,b) twice

We don't need local nlbufsiz (skb size) as nfulnl_alloc_skb() takes
the maximum anyway.

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>