vos: convertROtoRW may create two RW volumes

vos: convertROtoRW may create two RW volumes

If the RW volume is listed after the RO convert target in the VLDB,
the code failed to detect that an RW is already present and would
create a second RW volume.

Reviewed-on: http://gerrit.openafs.org/7385
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 9a728fd86c7add13e15dfa0d3062fa94cc77c53f)

Change-Id: I7e4eaf0ffefedb3c665e28656a75959eb8677071
Reviewed-on: http://gerrit.openafs.org/7497
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>