Windows: buf_RDRBuffersExist and cm_directIO

If 'cm_directIO' is true, then no RDR buffers exist.  Return immediately,
do not pass 'Go', and do not lock and walk the buffer hash table.

Change-Id: I9c6499d429d7591c8bd67ce3077ff4c4e7d6d62d
Reviewed-on: http://gerrit.openafs.org/9637
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>