Windows: Do not reference cm_data.rootSCachep directly

Windows: Do not reference cm_data.rootSCachep directly

cm_data.rootSCachep is a global pointer to the cm_scache_t that
represents the root.afs volume root directory.  Throughout the
code this cm_scache_t was being used without ensuring that a
callback to the volume is in fact valid.

Under most circumstances this would not be a problem.  However,
it is possible for a request to fail due to the lack of a callback
at a critical moment.  Add a new function cm_rootScachep() that
attempts to ensure that a callback is present (if possible) prior
to use the cm_scache_t object.

LICENSE MIT

Change-Id: I997dc05a2214788d32727c261947206a61cd8d32
Reviewed-on: http://gerrit.openafs.org/2491
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>