Windows: Set dirty page limit to 2 x ChunkSize / 4096

4096 is the size of a memory manager page.  ChunkSize is in octets.
Windows doesn't wait until the limit is reached to begin flushing
and ChunkSize is not a hard limit.

Change-Id: Ic1d5dda18eefbf309696b8a19ef5623248f54faf
Reviewed-on: http://gerrit.openafs.org/8965
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>