xen-netfront: avoid crashing on resume after a failure in talk_to_netback()

Unavoidable crashes in netfront_resume() and netback_changed() after a
previous fail in talk_to_netback() (e.g. when we fail to read MAC from
xenstore) were discovered. The failure path in talk_to_netback() does
unregister/free for netdev but we don't reset drvdata and we try accessing
it after resume.

Fix the bug by removing the whole xen device completely with
device_unregister(), this guarantees we won't have any calls into netfront
after a failure.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netfront.c

@@ -1934,8 +1934,7 @@ static int talk_to_netback(struct xenbus_device *dev,
 	xennet_disconnect_backend(info);
 	xennet_destroy_queues(info);
  out:
-	unregister_netdev(info->netdev);
-	xennet_free_netdev(info->netdev);
+	device_unregister(&dev->dev);
 	return err;
 }