rgw: fix radosgw-admin buckets list

Fixes: #5455
Backport: cuttlefish
This commit fixes a regression, where radosgw-admin buckets list
operation wasn't returning any data.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>