Zoekt: Fix negative unclaimed storage bytes

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175561



Merged-by: Terri Chu <tchu@gitlab.com>
Approved-by: Terri Chu <tchu@gitlab.com>
Reviewed-by: Dmitry Gruzd <dgruzd@gitlab.com>
Reviewed-by: Terri Chu <tchu@gitlab.com>
Co-authored-by: Dmitry Gruzd <dgruzd@gitlab.com>