rgw: convert bucket info if needed

Fixes: #7110
In dumpling, the bucket info was separated into bucket entry point and
bucket instance objects. When setting bucket attrs we only ended up
updating the bucket instance object. However, pre-dumpling buckets still
keep everything at the entry-point object, so acl changes didn't affect
anything (because we never updated the entry point). This change just
converts the bucket info into the new format.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit a5f8cc7ec9ec8bef4fbc656066b4d3a08e5b215b)