rbd: don't NULL out ->obj_request in rbd_img_obj_parent_read_full()

If rbd_img_request_submit() fails, parent_request->obj_request is
NULLed out, triggering an assert in rbd_obj_request_put():

  rbd_img_request_put(parent_request)
    rbd_parent_request_destroy
      rbd_obj_request_put(NULL)

Just remove it -- parent_request->obj_request will be put in
rbd_parent_request_destroy().
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -2682,8 +2682,6 @@ static int rbd_img_obj_parent_read_full(struct rbd_obj_request *obj_request)
 
 	parent_request->copyup_pages = NULL;
 	parent_request->copyup_page_count = 0;
-	parent_request->obj_request = NULL;
-	rbd_obj_request_put(obj_request);
 out_err:
 	if (pages)
 		ceph_release_page_vector(pages, page_count);