volscan: index offset output column

Add index offset as a possible volscan output column.

Reviewed-on: http://gerrit.openafs.org/6980
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 03ec768fa65dc5961e161517871f3035df70b643)

Change-Id: I17cede564fde259e04b7f1cb78724798808fb881
Reviewed-on: http://gerrit.openafs.org/11273
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>