Linux 3.8: vmtruncate removal

vmtruncate had been deprecated for a while and has now been
removed.  Do things the new way based on truncate_setsize.

Change-Id: If584329f7330f44be3909e4c8b935e4fe5e2b536
Reviewed-on: http://gerrit.openafs.org/8906
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>