net: Convert l2tp_net_ops

Init method is rather simple. Exit method queues del_work
for every tunnel from per-net list. This seems to be safe
to be marked async.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -1787,6 +1787,7 @@ static struct pernet_operations l2tp_net_ops = {
 	.exit = l2tp_exit_net,
 	.id   = &l2tp_net_id,
 	.size = sizeof(struct l2tp_net),
+	.async = true,
 };
 
 static int __init l2tp_init(void)