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>
(cherry picked from commit b1976dd00f5b29c01791272f63a18250319f2edb)