Windows: .readonly space reporting

Windows: .readonly space reporting

.readonly volumes can be stored on multiple partitions which each
have a different size and free space available.  To ensure consistency
regardless of which partition the .readonly is accessed from and
because .readonly volume sizes cannot change, report the allocation
size of the volume as the partition size.  Continue to report that
volume has zero free sectors because the volume cannot be written to.

Change-Id: Ib57b1175da048344af615b9350d72d9eb3ffe97d
Reviewed-on: http://gerrit.openafs.org/8976
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>