Windows: buf_CleanAsync scp->fid == bp->fid

Windows: buf_CleanAsync scp->fid == bp->fid

If buf_CleanAsync or buf_CleanAsyncLocked are called
with a non-NULL cm_scache_t parameter, that status object's
fid must be the same as the associated cm_buf_t object.
If not, the wrong locks will be held.

If the cm_scache_t parameter is NULL and cm_FindSCache()
returns NULL, it means that the cm_scache_t object associated
with the bp->fid has been flushed from the cache.  cm_GetSCache()
must therefore be called to allocate a new status object for the
FID.  If the status object cannot be allocated, then any dirty
data stored in the buffer will be discarded.

Change-Id: Ie5d4eb8a1090d4b3c0753b7ddee2de0799485a2e
Reviewed-on: http://gerrit.openafs.org/3604
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>