vol: use header access macros everywhere

a couple direct references to diskstuff have crept in. push them
back out

Change-Id: I66c7a157aeb326dee17012d551c0f499bae35bc7
Reviewed-on: http://gerrit.openafs.org/8807
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>