allow cloning of any volume to any volume with same parent ID

allow cloning of any volume to any volume with same parent ID

remove checks to disallow cloning of ro volumes to rw volumes,
which allows cloning of any volume within the same parent ID
grouping, including allowing destruction of newer version of the
volumes.

Change-Id: I13f600c33934b9af8237f049225d6bacfbbb5690
Reviewed-on: http://gerrit.openafs.org/5049
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>