libceph: export osd_req_op_data() macro

We already have one exported wrapper around it for extent.osd_data and
rbd_object_map_update_finish() needs another one for cls.request_data.

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