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>
(cherry picked from commit 9968b938b5d47fdf3a67db134bd2ea6bf3a28086)