Objecter: keep ObjectOperation rval pointers for writes

Objecter: keep ObjectOperation rval pointers for writes

Just before sending an op, prepare_mutate_op() is called, creating a
new Op. prepare_read_op() already copied over all the out-params
correctly, but for write operations the individual op return value
pointers were not copied, so they would not be filled in. With this
fixed, librados users can get the per-op return codes again.

Partially fixes: #6483
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>