bridge: dont send notification when skb->len == 0 in rtnl_bridge_notify

Reported in: https://bugzilla.kernel.org/show_bug.cgi?id=92081

This patch avoids calling rtnl_notify if the device ndo_bridge_getlink
handler does not return any bytes in the skb.

Alternately, the skb->len check can be moved inside rtnl_notify.

For the bridge vlan case described in 92081, there is also a fix needed
in bridge driver to generate a proper notification. Will fix that in
subsequent patch.

v2: rebase patch on net tree

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2895,11 +2895,15 @@ static int rtnl_bridge_notify(struct net_device *dev, u16 flags)
 			goto errout;
 	}
 
+	if (!skb->len)
+		goto errout;
+
 	rtnl_notify(skb, net, 0, RTNLGRP_LINK, NULL, GFP_ATOMIC);
 	return 0;
 errout:
 	WARN_ON(err == -EMSGSIZE);
 	kfree_skb(skb);
+	if (err)
 		rtnl_set_sk_err(net, RTNLGRP_LINK, err);
 	return err;
 }