use V_creationDate in DumpHeader for R/O volumes

use V_creationDate in DumpHeader for R/O volumes

This patch modifies a patch committed as 1e6fb1b7b7, the dumpTimes.to is now
set to creationDate for R/O volumes. The old value copyDate is wrong, if the
R/O volumes is re-cloned. This does not happen with "vos dump -clone", but
may happen with dumping a R/O volume directly: "vos dump <R/O volume>".

Reviewed-on: http://gerrit.openafs.org/11468
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit cb6de07fb8a12199ad0f1c4990f19074a9a54fcc)

Change-Id: I65df02bc4f98d58ba1461d36e79be2f72098ef7c
Reviewed-on: http://gerrit.openafs.org/11825
Reviewed-by: Daria Phoebe Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Hans-Werner Paulsen <hans@mpa-garching.mpg.de>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>