simplefs: fix doneReadWriteOp lock
simplefs: fix doneReadWriteOp lock

Suggested by songgao.

Issue: #1481

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