Add code for locking individual volumes on disk

Add code for locking individual volumes on disk

This adds the necessary APIs and associated changes to lock (on disk) an
individual volume on a particular partition. Nothing yet calls these new
functions.

Change-Id: Ibfa00293e7411f3f48eabdecb13b7e80e126a1ff
Reviewed-on: http://gerrit.openafs.org/1405
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>