messages/MOSDOpReply: assert no write result payloads

messages/MOSDOpReply: assert no write result payloads

Historically, we have cleared any outdata for writes.  Soon we will start
allowing these by recording them in the pglog.  First, let's assert these
are empty to ensure that aren't any misbehaving users we don't know about.

We only assert the *write* ops have no output buffer, since there may be
a compound op that is preceded by read ops.

Signed-off-by: Sage Weil <sage@redhat.com>