rgw: fix stripe_size calculation

Fixes: #8299
Backport: firefly
The stripe size calculation was broken, specifically affected cases
where we had manifest that described multiple parts.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1780 from ceph/wip-8299

rgw: fix stripe_size calculation

Reviewed-by: Sage Weil <sage@inktank.com>