net: dsa: deny enslaving 802.1Q upper to VLAN-aware bridge from PRECHANGEUPPER

There doesn't seem to be any strong technical reason for doing it this
way, but we'll be adding more checks for invalid upper device
configurations, and it will be easier to have them all grouped under
PRECHANGEUPPER.

Tested that it still works:
ip link set br0 type bridge vlan_filtering 1
ip link add link swp2 name swp2.100 type vlan id 100
ip link set swp2.100 master br0
[   20.321312] br0: port 5(swp2.100) entered blocking state
[   20.326711] br0: port 5(swp2.100) entered disabled state
Error: dsa_core: Cannot enslave VLAN device into VLAN aware bridge.
[   20.346549] br0: port 5(swp2.100) entered blocking state
[   20.351957] br0: port 5(swp2.100) entered disabled state
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/slave.c

@@ -1932,9 +1932,14 @@ static int dsa_slave_netdevice_event(struct notifier_block *nb,
 {
 	struct net_device *dev = netdev_notifier_info_to_dev(ptr);
 
-	if (event == NETDEV_CHANGEUPPER) {
+	switch (event) {
+	case NETDEV_PRECHANGEUPPER:
 		if (!dsa_slave_dev_check(dev))
 			return dsa_slave_upper_vlan_check(dev, ptr);
+		break;
+	case NETDEV_CHANGEUPPER:
+		if (!dsa_slave_dev_check(dev))
+			return NOTIFY_DONE;
 
 		return dsa_slave_changeupper(dev, ptr);
 	}