[NETFILTER]: nfnetlink_log: fix computation of netlink skb size

This patch is similar to nfnetlink_queue fixes. It fixes the computation
of skb size by using NLMSG_SPACE instead of NLMSG_ALIGN.

Signed-off-by: Eric Leblond <eric@inl.fr>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>