volser: Break callbacks to the target of VolClone

With the "-stayup" release mechanism, clients may have callbacks
to the target of VolClone rather than the target of VolRestore,
so also break callbacks there.

This could cause clients to not be notified of a volume release
done with -stayup and have stale contents.

Change-Id: I94009f4b9382471a3615d2a729e4ee3955a14d44
Reviewed-on: http://gerrit.openafs.org/11619
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>