rgw: reset objv tracker on bucket recreation

Fixes: #6951
If we cannot create a new bucket (as it already existed), we need to
read the old bucket's info. However, this was failing as we were holding
the objv tracker that we created for the bucket creation. We need to
clear it, as subsequent read using it will fail.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 859ed33ed7f9a96f4783dfb3e130d5eb60c622dd)
Merge pull request #1519 from ceph/wip-6951-dumpling

rgw: reset objv tracker on bucket recreation