Make rbd_obj_handle_read() look like a state machine and get rid of
the necessity to patch result in rbd_obj_handle_request(), completing
the removal of obj_req->xferred and img_req->xferred.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>