macvtap: check minor when unregistering

macvtap_device_event(NETDEV_UNREGISTER) should check vlan->minor to
determine if it is being invoked in the context of a macvtap_newlink
that failed, for example in this code sequence:

macvtap_newlink
  macvlan_common_newlink
    register_netdevice
      call_netdevice_notifiers(NETDEV_REGISTER, dev)
        macvtap_device_event(NETDEV_REGISTER)
          <fail here, vlan->minor = 0>
      rollback_registered(dev);
        rollback_registered_many
          call_netdevice_notifiers(NETDEV_UNREGISTER, dev);
            macvtap_device_event(NETDEV_UNREGISTER)
              <nothing to clean up here>
Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvtap.c

@@ -1303,6 +1303,9 @@ static int macvtap_device_event(struct notifier_block *unused,
 		}
 		break;
 	case NETDEV_UNREGISTER:
+		/* vlan->minor == 0 if NETDEV_REGISTER above failed */
+		if (vlan->minor == 0)
+			break;
 		devt = MKDEV(MAJOR(macvtap_major), vlan->minor);
 		device_destroy(macvtap_class, devt);
 		macvtap_free_minor(vlan);