qcow2: fix condition in is_zero_cluster

We should check for (res & BDRV_BLOCK_ZERO) only. The situation when we
will have !(res & BDRV_BLOCK_DATA) and will not have BDRV_BLOCK_ZERO is
not possible for images with bdi.unallocated_blocks_are_zero == true.

For those images where it's false, however, it can happen and we must
not consider the data zeroed then or we would corrupt the image.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -2412,7 +2412,7 @@ static bool is_zero_cluster(BlockDriverState *bs, int64_t start)
     BlockDriverState *file;
     int64_t res = bdrv_get_block_status_above(bs, NULL, start,
                                               s->cluster_sectors, &nr, &file);
-    return res >= 0 && ((res & BDRV_BLOCK_ZERO) || !(res & BDRV_BLOCK_DATA));
+    return res >= 0 && (res & BDRV_BLOCK_ZERO);
 }
 
 static bool is_zero_cluster_top_locked(BlockDriverState *bs, int64_t start)