rbd: factor out __rbd_osd_setup_discard_ops()

With obj_req->xferred removed, obj_req->ex.oe_off and obj_req->ex.oe_len
can be updated if required for alignment.  Previously the new offset and
length weren't stored anywhere beyond rbd_obj_setup_discard().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>