folder_block_manager: nits suggested by @akalin-keybase
folder_block_manager: nits suggested by @akalin-keybase

Issue: #723

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