Make external NFS clients know when files have their attributes changed and

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.

Reported by:	Kevin Buhr <buhr@asaurus.net>
Reviewed by:	delphij
Approved by:	re (gjb), delphij
Obtained from:	r272467

git-svn-id: svn://svn.freebsd.org/base/releng/10.1@273122 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f