Windows: Skip Extent operations if Direct IO

If the redirector is using Direct IO servicing there are no extents
in use.  Skip the AFSFlushExtents, AFSTearDownExtents, and related
calls unless extent processing is in use.  This will reduce lock
contention and reduce cpu processing.

Reviewed-on: http://gerrit.openafs.org/10215
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 6de822d647c2d2707f4f0af149579101dd0071f8)

Change-Id: I68c3ac0ae6230efb8b7439c55c0415856105739a
Reviewed-on: http://gerrit.openafs.org/10220
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>