libfs: clean subdir in `NewFS` if needed
libfs: clean subdir in `NewFS` if needed

Suggested by jzila.

Issue: #1260

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