batman-adv: Allow to modify slaves of soft-interfaces through rntl_link

The sysfs configuration interface of batman-adv to add/remove slaves of an
soft-iface is not deadlock free and doesn't follow the currently common way to
modify slaves of an interface.

An additional configuration interface though rtnl_link is introduced which
provides easy device adding/removing with tools like "ip":
$ ip link set dev eth0 master bat0
$ ip link set dev eth0 nomaster

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
diff --git a/net/batman-adv/hard-interface.c b/net/batman-adv/hard-interface.c
index 6c32607..f33ced8 100644
--- a/net/batman-adv/hard-interface.c
+++ b/net/batman-adv/hard-interface.c
@@ -350,9 +350,13 @@
 	hard_iface->soft_iface = soft_iface;
 	bat_priv = netdev_priv(hard_iface->soft_iface);
 
+	ret = netdev_master_upper_dev_link(hard_iface->net_dev, soft_iface);
+	if (ret)
+		goto err_dev;
+
 	ret = bat_priv->bat_algo_ops->bat_iface_enable(hard_iface);
 	if (ret < 0)
-		goto err_dev;
+		goto err_upper;
 
 	hard_iface->if_num = bat_priv->num_ifaces;
 	bat_priv->num_ifaces++;
@@ -362,7 +366,7 @@
 		bat_priv->bat_algo_ops->bat_iface_disable(hard_iface);
 		bat_priv->num_ifaces--;
 		hard_iface->if_status = BATADV_IF_NOT_IN_USE;
-		goto err_dev;
+		goto err_upper;
 	}
 
 	hard_iface->batman_adv_ptype.type = ethertype;
@@ -401,7 +405,10 @@
 out:
 	return 0;
 
+err_upper:
+	netdev_upper_dev_unlink(hard_iface->net_dev, soft_iface);
 err_dev:
+	hard_iface->soft_iface = NULL;
 	dev_put(soft_iface);
 err:
 	batadv_hardif_free_ref(hard_iface);
@@ -450,6 +457,7 @@
 	if (!bat_priv->num_ifaces && autodel == BATADV_IF_CLEANUP_AUTO)
 		batadv_softif_destroy_sysfs(hard_iface->soft_iface);
 
+	netdev_upper_dev_unlink(hard_iface->net_dev, hard_iface->soft_iface);
 	hard_iface->soft_iface = NULL;
 	batadv_hardif_free_ref(hard_iface);