net: Convert iptable_nat_net_ops

These pernet_operations unregister net::ipv4::nat_table table.
Another net/pernet_operations do not send ipv4 packets to foreign
net namespaces. So, we mark them async.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>