rgw: fetch object attrs on multipart completion

Fixes: #8452
Backport: firefly
This fixes a regression following a code cleanup.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1896 from ceph/wip-8452

rgw: fetch object attrs on multipart completion

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>