- Support synchronous directio only. Defer asynchronous directio until
    it can be made safe.
  - If read/write exits due to an error, return number of bytes read/written
    prior to occurrence of the error.
  - Make sure we mark read pages as dirty in case we're doing zero-copy tricks.
    Export set_page_dirty_lock() for use by NFS directio.
  - Ensure we revalidate stale attribute info.