net: Convert ctnetlink_net_ops

These pernet_operations register and unregister
two conntrack notifiers, and they seem to be safe
to be executed in parallel.

General/not related to async pernet_operations JFI:
ctnetlink_net_exit_batch() actions are grouped in batch,
and this could look like there is synchronize_rcu()
is forgotten. But there is synchronize_rcu() on module
exit patch (in ctnetlink_exit()), so this batch may
be reworked as simple .exit method.
Signed-off-by: NKirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/nf_conntrack_netlink.c

@@ -3417,6 +3417,7 @@ static void __net_exit ctnetlink_net_exit_batch(struct list_head *net_exit_list)
 static struct pernet_operations ctnetlink_net_ops = {
 	.init		= ctnetlink_net_init,
 	.exit_batch	= ctnetlink_net_exit_batch,
+	.async		= true,
 };
 
 static int __init ctnetlink_init(void)