xen-blkfront: beyond ARRAY_SIZE of info->shadow

Do not go beyond ARRAY_SIZE of info->shadow
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/block/xen-blkfront.c

@@ -122,7 +122,7 @@ static DEFINE_SPINLOCK(blkif_io_lock);
 static int get_id_from_freelist(struct blkfront_info *info)
 {
 	unsigned long free = info->shadow_free;
-	BUG_ON(free > BLK_RING_SIZE);
+	BUG_ON(free >= BLK_RING_SIZE);
 	info->shadow_free = info->shadow[free].req.id;
 	info->shadow[free].req.id = 0x0fffffee; /* debug */
 	return free;