Windows: buf_ClearRDRFlag and cm_directIO

When 'cm_directIO' is true, there are no RDR held buffers therefore
there is nothing for buf_ClearRDRFlag() to do.  Do not pass 'Go',
do not obtain cm_scache.rw exclusive nor buf_globalLock shared.
Simply return immediately.

Change-Id: I4c6d1486e3a2bef987a3eb36d8de446e9009e0d2
Reviewed-on: http://gerrit.openafs.org/9638
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>