simplefs: use filepath.ToSlash so Windows paths are correct
simplefs: use filepath.ToSlash so Windows paths are correct

simplefs expects all paths standardized to /.

Issue: KBFS-2661

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