batman-adv: Use kref_get for batadv_gw_select

batadv_gw_select requires that the caller already has a valid reference for
new_gw_node. It is therefore not possible that it has an reference counter
of 0 and was still given to this function

The kref_get function instead WARNs (with debug information) when the
reference counter would still be 0. This makes a bug in batman-adv better
visible because kref_get_unless_zero would have ignored this problem.
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>

net/batman-adv/gateway_client.c

@@ -135,8 +135,8 @@ static void batadv_gw_select(struct batadv_priv *bat_priv,
 
 	spin_lock_bh(&bat_priv->gw.list_lock);
 
-	if (new_gw_node && !kref_get_unless_zero(&new_gw_node->refcount))
-		new_gw_node = NULL;
+	if (new_gw_node)
+		kref_get(&new_gw_node->refcount);
 
 	curr_gw_node = rcu_dereference_protected(bat_priv->gw.curr_gw, 1);
 	rcu_assign_pointer(bat_priv->gw.curr_gw, new_gw_node);