net: Convert sit_net_ops

These pernet_operations are similar to ip6_tnl_net_ops. Exit method
unregisters all net sit devices, and it looks like another
pernet_operations are not interested in foreign net sit list.
Init method registers netdevice. So, it's possible to mark them async.
Signed-off-by: NKirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -1878,6 +1878,7 @@ static struct pernet_operations sit_net_ops = {
 	.exit_batch = sit_exit_batch_net,
 	.id   = &sit_net_id,
 	.size = sizeof(struct sit_net),
+	.async = true,
 };
 
 static void __exit sit_cleanup(void)