vos: convertROtoRW susceptible to VLDB changes during override prompt

vos: convertROtoRW susceptible to VLDB changes during override prompt

vos convertROtoRW obtains a VLDB entry, then peforms some setup logic
(including a possible user prompt) before obtaining a volume lock.
This exposes the code to possible time-of-check/time-of-use issues.
After obtaining the volume lock, get a second copy of the VLDB entry
and compare it to the first copy; if it has changed, fail the conversion
with an error message asking the user to re-issue the vos convertROtoRW
command.

Change-Id: I9c1a634cea2e22188869d54b00b7831aed12b1cd
Reviewed-on: http://gerrit.openafs.org/8008
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>