xen-blkfront: Avoid that gcc 7 warns about fall-through when building with W=1

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Roger Pau Monn303251 <roger.pau@citrix.com>
Cc: xen-devel@lists.xenproject.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/xen-blkfront.c

@@ -2456,7 +2456,7 @@ static void blkback_changed(struct xenbus_device *dev,
 	case XenbusStateClosed:
 		if (dev->state == XenbusStateClosed)
 			break;
-		/* Missed the backend's Closing state -- fallthrough */
+		/* fall through */
 	case XenbusStateClosing:
 		if (info)
 			blkfront_closing(info);