osdc/Objecter: only make handlers set *prval if EIO

The normal Objecter handlers set *prval for each operation; we only need to
(re)set it if we have a special reason (like a decoding error).

Signed-off-by: Sage Weil <sage@inktank.com>
osdc/Objecter: clean up completion handlers that set *prval=0

If the OSD operations returns 0, these are unnecessary.  If it does not,
then these are misleading.  In both cases here, the OSD sets the result to
zero, so this has no client-visible change.r

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