vos: volume lock not released on convertROtoRW error

vos: volume lock not released on convertROtoRW error

If an error occurs during convertROtoRW, the volume lock may
not be released.

Reviewed-on: http://gerrit.openafs.org/7389
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 33eff6597facc01bbcd5266fb83c8a6bfa40dfa8)

Change-Id: I0b87fd3b99f178d99f997032c14673def111c1de
Reviewed-on: http://gerrit.openafs.org/7499
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>