rbd: incorporate ceph_object_extent

obj_req->object_no -> obj_req->ex.oe_objno
obj_req->offset -> obj_req->ex.oe_off
obj_req->length -> obj_req->ex.oe_len

... and use ex for linking object requests to image requests.

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