net: Convert gtp_net_ops

These pernet_operations are similar to bond_net_ops. Exit method
unregisters all net gtp devices, and it looks like another
pernet_operations are not interested in foreign net gtp list.
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>

drivers/net/gtp.c

@@ -1325,6 +1325,7 @@ static struct pernet_operations gtp_net_ops = {
 	.exit	= gtp_net_exit,
 	.id	= &gtp_net_id,
 	.size	= sizeof(struct gtp_net),
+	.async	= true,
 };
 
 static int __init gtp_init(void)