Windows: No RO volume test SetFilePosInfo

Setting the file position information on a file is not a data
changing operation.  Do not perform a readonly volume check.

Change-Id: I5dccff569b39187c2891d4339f18db8c54c029a7
Reviewed-on: http://gerrit.openafs.org/11066
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>