rgw: reverse logic to identify next part

Fixes: #7935
The check that identifies whether we need to move to the next part when
iterating over an object was reversed.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1577 from ceph/wip-7935

rgw: reverse logic to identify next part

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>