radosgw-admin: fix object policy read op

Fixes: #7083
This was broken when we fixed #6940. We use the same function to both
read the bucket policy and the object policy. However, each needed to be
treated differently. Restore old behavior for objects.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1050 from ceph/wip-7083

radosgw-admin: fix object policy read op

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>