rbd: zero return code in rbd_dev_image_id()

When rbd_dev_probe() calls rbd_dev_image_id() it expects to get
a 0 return code if successful, but it is getting a positive value.

The reason is that rbd_dev_image_id() returns the value it gets from
rbd_req_sync_exec(), which returns the number of bytes read in as a
result of the request.  (This ultimately comes from
ceph_copy_from_page_vector() in rbd_req_sync_op()).

Force the return value to 0 when successful in rbd_dev_image_id().
Do the same in rbd_dev_v2_object_prefix().
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>

drivers/block/rbd.c

@@ -2207,6 +2207,7 @@ static int rbd_dev_v2_object_prefix(struct rbd_device *rbd_dev)
 	dout("%s: rbd_req_sync_exec returned %d\n", __func__, ret);
 	if (ret < 0)
 		goto out;
+	ret = 0;    /* rbd_req_sync_exec() can return positive */
 
 	p = reply_buf;
 	rbd_dev->header.object_prefix = ceph_extract_encoded_string(&p,
@@ -2900,6 +2901,7 @@ static int rbd_dev_image_id(struct rbd_device *rbd_dev)
 	dout("%s: rbd_req_sync_exec returned %d\n", __func__, ret);
 	if (ret < 0)
 		goto out;
+	ret = 0;    /* rbd_req_sync_exec() can return positive */
 
 	p = response;
 	rbd_dev->image_id = ceph_extract_encoded_string(&p,