vlan: allow to change type when no vlan device is hooked on netdev

vlan_info might be present but still no vlan devices might be there.
That is in case of vlan0 automatically added.

So in that case, allow to change netdev type.
Reported-by: Jon Stanley <jstanley@rmrf.net>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/8021q/vlan.c

@@ -463,7 +463,9 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
 
 	case NETDEV_PRE_TYPE_CHANGE:
 		/* Forbid underlaying device to change its type. */
-		return NOTIFY_BAD;
+		if (vlan_uses_dev(dev))
+			return NOTIFY_BAD;
+		break;
 
 	case NETDEV_NOTIFY_PEERS:
 	case NETDEV_BONDING_FAILOVER: