batman-adv: use __dev_get_by_index instead of dev_get_by_index to find interface

The following call chains indicate that batadv_is_on_batman_iface()
is always under rtnl_lock protection as call_netdevice_notifier()
is protected by rtnl_lock. So if __dev_get_by_index() rather than
dev_get_by_index() is used to find interface handler in it, this
would help us avoid to change interface reference counter.

call_netdevice_notifier()
  batadv_hard_if_event()
    batadv_hardif_add_interface()
      batadv_is_valid_iface()
        batadv_is_on_batman_iface()

Cc: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: NYing Xue <ying.xue@windriver.com>
Acked-by: NAntonio Quartulli <antonio@meshcoding.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/batman-adv/hard-interface.c

@@ -87,15 +87,13 @@ static bool batadv_is_on_batman_iface(const struct net_device *net_dev)
 		return false;
 
 	/* recurse over the parent device */
-	parent_dev = dev_get_by_index(&init_net, net_dev->iflink);
+	parent_dev = __dev_get_by_index(&init_net, net_dev->iflink);
 	/* if we got a NULL parent_dev there is something broken.. */
 	if (WARN(!parent_dev, "Cannot find parent device"))
 		return false;
 
 	ret = batadv_is_on_batman_iface(parent_dev);
 
-	if (parent_dev)
-		dev_put(parent_dev);
 	return ret;
 }