netfilter: update rwlock initialization for nat_table

    
The commit e099a173
(netfilter: netns nat: per-netns NAT table) renamed the
nat_table from __nat_table to nat_table without updating the
__RW_LOCK_UNLOCKED(__nat_table.lock).

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>