folder_update_prepper: fix vet and clarify lock name
folder_update_prepper: fix vet and clarify lock name

Suggested by jzila.

Issue: #962

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client