vos: convertROtoRW does not respect volume lock

vos: convertROtoRW does not respect volume lock

vos convertROtoRW was not checking the return code from
ubik_VL_SetLock().

Reviewed-on: http://gerrit.openafs.org/7388
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 38ebd79bafee3c97323aa603906d1d5f279ec450)

Change-Id: I391c5b0d3d385af08eb2aec245f943081c7b84e3
Reviewed-on: http://gerrit.openafs.org/7498
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>