use V_copyDate in DumpHeader for cloned volumes

Volume dumps can be created from backup volumes, cloned volumes, or
directly from RW volumes. The beginning and end of the time range
covered by the dump is recorded in the DumpHeader. The end time is
based on the type of the volume. Use backupDate for backup volumes,
use copyDate for cloned volumes, and updateDate for RW volumes.

Change-Id: I18206d25f056e553eed2f3c3e0695fed003f3714
Reviewed-on: http://gerrit.openafs.org/11389
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>