Windows: check perms before RXAFS_GetVolumeStatus

Windows: check perms before RXAFS_GetVolumeStatus

Instead of calling RXAFS_GetVolumeStatus naked, perform a read
permission check using RXAFS_FetchStatus first.  This permits EACCES
caching to prevent unnecessary requests.

Regardless of which FileId is queried, always use the root vnode
FileId for the permission check.  The file server performs its
permission check using the root vnode.

Change-Id: I3260bf0061beed5d95aae1d40e25d17be1811271
Reviewed-on: http://gerrit.openafs.org/7641
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>