To spare checking for "this reply fits into a page, but does it fit
into my buffer?" in some callers, osd_req_op_cls_response_data_pages()
needs to know how big it is.
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
Reviewed-by: NJason Dillaman <dillaman@redhat.com>