netfilter: nfnetlink_log: add net namespace support for nfnetlink_log

This patch makes /proc/net/netfilter/nfnetlink_log pernet.
Moreover, there's a pernet instance table and lock.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>