net: Convert cfg802154_pernet_ops

These pernet_operations have only exit method, which
moves devices from cfg802154_rdev_list to init_net.
This may occur in any time from nl802154_wpan_phy_netns(),
so we are nice with rtnl_lock() synchronization.
Signed-off-by: NKirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: NStefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ieee802154/core.c

@@ -345,6 +345,7 @@ static void __net_exit cfg802154_pernet_exit(struct net *net)
 
 static struct pernet_operations cfg802154_pernet_ops = {
 	.exit = cfg802154_pernet_exit,
+	.async = true,
 };
 
 static int __init wpan_phy_class_init(void)