net: Convert nfnetlink_net_ops

These pernet_operations create and destroy net::nfnl
socket of NETLINK_NETFILTER code. There are no other
places, where such type the socket is created, except
these pernet_operations. It seem other pernet_operations
depending on CONFIG_NETFILTER_NETLINK send messages
to this socket. So, we mark it async.

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