vos: convertROtoRW incorrect warning when RO not in VLDB

vos: convertROtoRW incorrect warning when RO not in VLDB

vos convertROtoRW will issue an incorrect warning about a partition
mismatch if the RO to convert is not in the VLDB.  Only check the
partition if the RO is in the VLDB.

Change-Id: Ib2726bc5bf6697898ad26dc1d817143da3286ba3
Reviewed-on: http://gerrit.openafs.org/8036
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit b650106e301ecefe2f5c8f4a34081af1f5f74901)
Reviewed-on: http://gerrit.openafs.org/8215
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>