pybind/rados/rados: do not pass prval from stack

pybind/rados/rados: do not pass prval from stack

The prval is a pointer to an int to write the final completion code of
the rados op.  This can't be on the stack since we immediately leave the
current scope after preparing the op (looong before we do the rados op).

We keep the tuple return value to avoid breaking users of this API
(devicehealth module, gnocchi at a minimum).

Fixes: http://tracker.ceph.com/issues/25175
Signed-off-by: Sage Weil <sage@redhat.com>