rados write returns (error code, data)

Hi all, I've just merged https://github.com/ceph/ceph/pull/30191 to master, which adds assertions in the OSD that the result of any successful write is zero (or a negate error), and the output buffer is empty. This is preparation for adding support for >0 and non-empty buffers--but first we should verify no existing code is returning such data inadvertantly. If this turns up failures in the rgw, rbd, or cephfs suites, it's probably because a cls method is returning >0 or data for a write that the OSD was previously clearing out on its behalf. The fix is to just adjust the cls code to return 0 and no data. Thanks! sage