Windows: dirty threshold 64 * chunksize

must be large enough to keep the pipe full

Change-Id: I1bf8b5800ff286e74dbe57ced19943db891b715f
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>