xen/blkfront: allow xenbus state transition to Closing->Closed when not Connected

This situation can occur when attempting to attach a block device whose
backend is an empty physical CD-ROM driver. The backend in this case
will go directly from the Initialising state to Closing->Closed.
Previously this would result in a NULL pointer deref on info->gd
(xenbus_dev_fatal does not return as a1a15ac5 seems to expect)

Cc: stable@kernel.org
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/block/xen-blkfront.c

@@ -977,8 +977,10 @@ static void backend_changed(struct xenbus_device *dev,
 		break;
 
 	case XenbusStateClosing:
-		if (info->gd == NULL)
-			xenbus_dev_fatal(dev, -ENODEV, "gd is NULL");
+		if (info->gd == NULL) {
+			xenbus_frontend_closed(dev);
+			break;
+		}
 		bd = bdget_disk(info->gd, 0);
 		if (bd == NULL)
 			xenbus_dev_fatal(dev, -ENODEV, "bdget failed");