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>