rbd: simplify rbd_osd_req_create()

No need to pass rbd_dev and op_type to rbd_osd_req_create(): there are
no standalone (!IMG_DATA) object requests anymore and osd_req->r_flags
can be set in rbd_osd_req_format_{read,write}().

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