MFC: r253506
The NFSv4 server incorrectly assumed that the high order words of
the attribute bitmap argument would be non-zero. This caused an
interoperability problem for a recent patch to the Linux NFSv4 client.
The Linux folks have changed their patch to avoid this, but this
patch fixes the problem on the server.

Reported and tested by:	a.heider@gmail.com (Andre Heider)
Approved by:	re (Xin Li)