rbd: change rbd_obj_request_submit() signature

- osdc parameter is useless
- starting with commit 5aea3dcd ("libceph: a major OSD client
  update"), ceph_osdc_start_request() always returns success

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: David Disseldorp <ddiss@suse.de>