Make external NFS clients know when files have their attributes changed and
avoid cache the file's state indefinitely. The va_filerev is what is sent
to the client as the "change" attribute, the client is periodically fetching
the attributes and without this option the attribute remains as some garbage
value.

Phabric:	D905
Reported by:	Kevin Buhr <buhr@asaurus.net>
Reviewed by:	rmacklem, delphij
Approved by:	delphij
Obtained from:	r272467
Sponsored by:	QNAP Systems Inc.

(cherry picked from commit 6b6a0e5b05ab1414054afc77562682367d8856fc)
(cherry picked from commit d606af049e52eb02a06b9d67ab4e04c497f94ecc)