vos: move convertROtoRW core logic to vsprocs

Create new vsprocs routine UV_ConvertRO in preparation for adding
new function to vos convertROtoRW.

Change-Id: Ic66ecbf7cacb277891bec9f8783040995ce6ce17
Reviewed-on: http://gerrit.openafs.org/9277
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>