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.

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>
(cherry picked from commit d0479bbaf43900d6733c3f7517926ee9813c9610)

Change-Id: Iafc64e19544dec9fcaefad5ad274eac4133f4083
Reviewed-on: http://gerrit.openafs.org/8942
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>