vos: Do not try to remove backup volume id 0

vos: Do not try to remove backup volume id 0

Currently we always try to delete a BK volume if we're deleting the
RW. If the BK volume id is 0, this is never going to do anything, so
don't try to delete it.

Reviewed-on: http://gerrit.openafs.org/7140
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 46bdb318c45b4e93a5b297e6fdee2b2acbed4b55)

Change-Id: I6d5406d6e39513d3f38e7674fa726b627ca5f1f0
Reviewed-on: http://gerrit.openafs.org/7935
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>