Windows: optimize buf_FlushCleanPages

Windows: optimize buf_FlushCleanPages

If the pages to be flushed are from a readonly or backup volume
they can't be dirty.  In this case there is no need to stabilize
the pages before they are locked and recycled.

Stablilization is performed on the cm_scache_t object so do not
stabilize and unstablize for each cm_buf_t object.

LICENSE MIT

Change-Id: I6fa86cf0a33dc9f45d77911909e799c0fc6510cf
Reviewed-on: http://gerrit.openafs.org/2518
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>