xen-netback: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/xenbus.c

@@ -655,7 +655,7 @@ static void frontend_changed(struct xenbus_device *dev,
 		set_backend_state(be, XenbusStateClosed);
 		if (xenbus_dev_is_online(dev))
 			break;
-		/* fall through if not online */
+		/* fall through - if not online */
 	case XenbusStateUnknown:
 		set_backend_state(be, XenbusStateClosed);
 		device_unregister(&dev->dev);