staging/lustre: Update FID documentation link.

When OpenSFS took over lustre.org, there was some reshuffling.
FIDs on ZFS document is now at
http://wiki.old.lustre.org/index.php/Architecture_-_Interoperability_fids_zfs


instead of the old location, so update comments accordingly.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reported-by: Xose Vazquez Perez <xose.vazquez@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>