crimson/osd: remove osd_op_params_t::user_at_version

crimson/osd: remove osd_op_params_t::user_at_version

osd_op_params_t::user_at_version was populated from
osd_op_params_t::at_version before the call to prepare_transaction,
which incremented osd_op_params_t::at_version.version.  As a result,
the value stored in object_info_t::user_version ended up one version
behind object_info_t::version.  The log entry, on the other hand,
ended up with the correct version as OpsExecutor::prepare_transaction
populates it directly from at_version.  As a result, the primary could
return different versions to the client depending on whether the IO was
already in the log.

This commit eliminates osd_op_params_t::user_at_version and updates
PGBackend::mutate_object to behave like prepare_transaction.  Because
the prior commit removes the prepare_transaction increment, this isn't
strictly necessary, but it is simpler.

Fixes: https://tracker.ceph.com/issues/65857
Signed-off-by: Samuel Just <sjust@redhat.com>