Windows: Restore "fs getcacheparms" used space

Windows: Restore "fs getcacheparms" used space

buf_Init() adds all of the available buffers to the free queue
at startup and cm_data.buf_freeCount tracks the number of items
in the free queue.  So it cannot be used as a method of reporting
how much of the cache space has been used.   Add a new buf_usedCount
parameter to the cm_memmap data to track the number of cache blocks
that have been used up to the total number of allocated blocks.
buf_usedCount can then be used in cm_IoctlGetCacheParms to restore
the original behavior.

Change-Id: I3ecdfdbda1c423551b7e7691243696e0f1553edc
Reviewed-on: http://gerrit.openafs.org/8578
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>