Windows: Decr. used buf count flush and deleted files

Windows: Decr. used buf count flush and deleted files

When flushing the cache (fs flush*) or when deleting files or
when a file server reported error indicates that the buffer is
bad, decrement the used cache count so "fs getcacheparms" now
indicates a floating value based upon the number of buffers
containing potentially valid data.

Change-Id: I1bfb7b64ff36832526a02e9374ab87baf679f100
Reviewed-on: http://gerrit.openafs.org/8579
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>