net: Convert vti6_net_ops

These pernet_operations are similar to ip6_tnl_net_ops. Exit method
unregisters all net vti6 tunnels, and it looks like another
pernet_operations are not interested in foreign net vti6 list.
Init method registers netdevice. So, it's possible to mark them async.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_vti.c

@@ -1148,6 +1148,7 @@ static struct pernet_operations vti6_net_ops = {
 	.exit_batch = vti6_exit_batch_net,
 	.id   = &vti6_net_id,
 	.size = sizeof(struct vti6_net),
+	.async = true,
 };
 
 static struct xfrm6_protocol vti_esp6_protocol __read_mostly = {