nfsd: update create verifier comment

I don't know if that Solaris behavior matters any more or if it's still
possible to look up that bug ID any more.  The XFS behavior's definitely
still relevant, though; any but the most recent XFS filesystems will
lose the top bits.

Reported-by: Frank S. Filz <ffilzlnx@mindspring.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>