net: dsa: allow port_bridge_join() to override extack message

Some drivers might report that they are unable to bridge ports by
returning -EOPNOTSUPP, but still wants to override extack message.
In order to do so, in dsa_slave_changeupper(), if port_bridge_join()
returns -EOPNOTSUPP, check if extack message is set and if so, do not
override it.
Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/slave.c

@@ -2460,8 +2460,9 @@ static int dsa_slave_changeupper(struct net_device *dev,
 			if (!err)
 				dsa_bridge_mtu_normalization(dp);
 			if (err == -EOPNOTSUPP) {
-				NL_SET_ERR_MSG_MOD(extack,
-						   "Offloading not supported");
+				if (!extack->_msg)
+					NL_SET_ERR_MSG_MOD(extack,
+							   "Offloading not supported");
 				err = 0;
 			}
 			err = notifier_from_errno(err);