batman-adv: Demote batadv-on-batadv skip error message

The error message "Cannot find parent device" was shown for users of
macvtap (on batadv devices) whenever the macvtap was moved to a different
netns. This happens because macvtap doesn't provide an implementation for
rtnl_link_ops->get_link_net.

The situation for which this message is printed is actually not an error
but just a warning that the optional sanity check was skipped. So demote
the message from error to warning and adjust the text to better explain
what happened.
Reported-by: Leonardo Mörlein <freifunk@irrelefant.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/hard-interface.c

@@ -165,9 +165,9 @@ static bool batadv_is_on_batman_iface(const struct net_device *net_dev)
 	/* recurse over the parent device */
 	parent_dev = __dev_get_by_index((struct net *)parent_net,
 					dev_get_iflink(net_dev));
-	/* if we got a NULL parent_dev there is something broken.. */
 	if (!parent_dev) {
-		pr_err("Cannot find parent device\n");
+		pr_warn("Cannot find parent device. Skipping batadv-on-batadv check for %s\n",
+			net_dev->name);
 		return false;
 	}