cls_rgw: update the appropriate prev key entry

We need to update the previous key entry that it's no longer the latest,
need to use the list entry, not the data entry (which only serves as a
pointer to the list entry).

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw, cls_rgw: cls_bucket_list returns raw key in map

This is needed so that we dump the objects in the required order (from
newest to oldest versions for each object). While we're at it, revert to
the original encoding of rgw_bucket_dir, which will be better for
backward compatibility.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>