Windows: cm_HaveCallback optimize .readonly

Windows: cm_HaveCallback optimize .readonly

If the cm_scache_t has a registered callback, there is no reason
to obtain a volume reference for a .readonly.  Just use the
callback.

Reviewed-on: http://gerrit.openafs.org/7716
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit f73aebbe8772e62bf8930d70e9edab2f82ff8984)

Change-Id: Ibacb272fade3aa24beb0b7c9a966b9e769385669
Reviewed-on: http://gerrit.openafs.org/8638
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>