net: Convert fou_net_ops

These pernet_operations initialize and destroy
pernet net_generic(net, fou_net_id) list.
The rest of net_generic(net, fou_net_id) accesses
may happen after netlink message, and in-tree
pernet_operations do not send FOU_GENL_NAME messages.
So, these pernet_operations are safe to be marked
as async.

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