dirty_bcache: don't increase buffer size on small writes
dirty_bcache: don't increase buffer size on small writes

Issue: KBFS-731

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