Windows: dirty threshold 64 * chunksize

must be large enough to keep the pipe full

Reviewed-on: http://gerrit.openafs.org/8996
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 3ecdfadded7cb6de6a0204cb1cf70ae764e17769)

Change-Id: I0560c39e2e64f8860d011d6e6cc0859e730aff26
Reviewed-on: http://gerrit.openafs.org/9057
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>