vos: Avoid creating volume with the same RO/BK ids

vos: Avoid creating volume with the same RO/BK ids

If we specified an RW id of 5, an RO id of 6, and no BK id, this code
would assign the BK id to RW+1, or 6. This gives the RO and BK volumes
the same volume id, which is a mistake. Choose a different id instead.

Change-Id: I7910008dd6147a95d482337f91926e9e0d924b9e
Reviewed-on: http://gerrit.openafs.org/7603
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>