rtnetlink: Fix regression in bridge VLAN configuration

Cited commit started returning errors when notification info is not
filled by the bridge driver, resulting in the following regression:

 # ip link add name br1 type bridge vlan_filtering 1
 # bridge vlan add dev br1 vid 555 self pvid untagged
 RTNETLINK answers: Invalid argument

As long as the bridge driver does not fill notification info for the
bridge device itself, an empty notification should not be considered as
an error. This is explained in commit 59ccaaaa49 ("bridge: dont send
notification when skb->len == 0 in rtnl_bridge_notify").

Fix by removing the error and add a comment to avoid future bugs.

Fixes: a8db57c1d2 ("rtnetlink: Fix missing error code in rtnl_bridge_notify()")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -4842,10 +4842,12 @@ static int rtnl_bridge_notify(struct net_device *dev)
 	if (err < 0)
 		goto errout;
 
-	if (!skb->len) {
-		err = -EINVAL;
+	/* Notification info is only filled for bridge ports, not the bridge
+	 * device itself. Therefore, a zero notification length is valid and
+	 * should not result in an error.
+	 */
+	if (!skb->len)
 		goto errout;
-	}
 
 	rtnl_notify(skb, net, 0, RTNLGRP_LINK, NULL, GFP_ATOMIC);
 	return 0;