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.

Reviewed-on: http://gerrit.openafs.org/7603
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 234bedae6ef506ec1ff53854a22548e892db3f11)

Change-Id: I332fed5b8cac23a847a8890188f899cff909f130
Reviewed-on: http://gerrit.openafs.org/9488
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>