use V_copyDate in DumpHeader for cloned volumes

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.

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>
(cherry picked from commit 1e6fb1b7b7ed32e2035452db9fc221f38a8b4956)

Change-Id: I7372a1f46b96c4c42fb45b71f0d71017216f3830
Reviewed-on: http://gerrit.openafs.org/11433
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Hans-Werner Paulsen <hans@mpa-garching.mpg.de>
Reviewed-by: Daria Phoebe Brashear <shadow@your-file-system.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>