vos release: Force full dump on RO_DONTUSE sites

vos release: Force full dump on RO_DONTUSE sites

When releasing a volume, currently we perform an incremental dump on
RO_DONTUSE sites if the volume exists on the remote site. Since
RO_DONTUSE implies that we do not expect a site to exist there, delete
the extant volume if we find one and force a full dump.

If we perform an incremental dump, we run the risk of incrementally
dumping to a temporary volume that the administrator is not aware of,
and doesn't have any actual data but has a last update time late
enough that it may be missing some data after the incremental dump. So
to avoid that, force a full dump every time.

Change-Id: If751a66d6e1499b5d6f67457ae28277a6200c53a
Reviewed-on: http://gerrit.openafs.org/2731
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>