rgw: don't overwrite bucket entry data when syncing user stats

Fixes: #7687
When syncing user bucket stats we overwritten the entire entry with the
passed in entry. We should only look at the stats portion, and not
overwrite the rest (which contains bucket creation time).

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1432 from ceph/wip-7687

rgw: don't overwrite bucket entry data when syncing user stats

Reviewed-by: Sage Weil <sage@inktank.com>