batman-adv: wait for rtnl in batadv_store_mesh_iface instead of failing if it is taken

The rtnl_lock in batadv_store_mesh_iface has been converted to a rtnl_trylock
some time ago to avoid a possible deadlock between rtnl and s_active on removal
of the sysfs nodes.

The behaviour introduced by that was quite confusing as it could lead to the
sysfs store to fail, making batman-adv setup scripts unreliable. As recently the
sysfs removal was postponed to a worker not running with the rtnl taken, the
deadlock can't occur any more and it is safe to change the trylock back to a
lock to make the sysfs store reliable again.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Reviewed-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>

net/batman-adv/sysfs.c

@@ -582,10 +582,7 @@ static ssize_t batadv_store_mesh_iface(struct kobject *kobj,
 	    (strncmp(hard_iface->soft_iface->name, buff, IFNAMSIZ) == 0))
 		goto out;
 
-	if (!rtnl_trylock()) {
-		ret = -ERESTARTSYS;
-		goto out;
-	}
+	rtnl_lock();
 
 	if (status_tmp == BATADV_IF_NOT_IN_USE) {
 		batadv_hardif_disable_interface(hard_iface,