From Trond:

  When the nanosecond resolution on [acm]time was introduced to 2.5.x,
the knfsd GETATTR responses were converted to make use of the
nanosecond field, but the pre/post WCC attributes were not. This will
lead to a lot of unnecessary cache invalidations on the clients.

The following trivial patch should fix up knfsd so that it stores and
encodes the full 'struct timespec' in both pre and post attribute
fields.