rbd: remove barriers from img_request_layered_{set,clear,test}()

IMG_REQ_LAYERED is set in rbd_img_request_create(), and tested and
cleared in rbd_img_request_destroy() when the image request is about to
be destroyed.  The barriers are unnecessary.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>