osd: do not return data payload for successful writes

osd: do not return data payload for successful writes

We were somewhat inadvertantly returning a data payload for write
operations.  This was a side-effect of the OpContext::ops field being a
reference to MOSDOp::ops: the return data would end up there, and then
the MOSDOpReply ctor would copy it.

Fix this by breaking the ref, and making the do_op() logic also claim
return result data for error values (so that errors can return data to the
caller).

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