Retrieve the post-operation attribute changes for NFSv4 READ and
WRITE operations. Unlike for NFSv2 and NFSv3, we do not retrieve the
full set of file attributes. The main reason for this is that
interpreting attributes is a much heavier task on NFSv4 (requiring, for
instance, translation of file owner names into uids ...). Hence

  For a READ request, we retrieve only the 'change attribute' (for cache
  consistency checking) and the atime.

  For a WRITE request, we retrieve the 'change attribute' and the file size.
  In addition, we retrieve the value of the change attribute prior to the
  write operation, in order to be able to do weak cache consistency checking.