vos: convertROtoRW may create 2nd RW on a fileserver

vos: convertROtoRW may create 2nd RW on a fileserver

If an RW is already present on disk on the target server (any partition),
'vos convertROtoRW' will still convert the RO, creating a second RW on the server.
Detect this and refuse to convert the RO by returning EXDEV (invalid cross-device link).

Change-Id: Ide15a7c39f2a975fd8597e497094b6a67b448e4f
Reviewed-on: http://gerrit.openafs.org/7934
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>