Windows: buf_CleanAsync is not async; rename it

Windows: buf_CleanAsync is not async; rename it

buf_CleanAsync() calls cm_BufWrite() which stores the dirty
buffers synchronously.  There is nothing asynchronous about
buf_CleanAsync() so rename it to buf_Clean() and buf_CleanAsyncLocked()
to buf_CleanLocked().   Update the comments to remove the references
to the asynchronous processing which doesn't exist.

That is not to say that the call to buf_Clean() in buf_GetNewLocked()
should not be asynchronous; it should.  There is no such functionality
at the moment.  One approach would be to modify buf_IncrSyncer to
trigger on an event set by buf_GetNewLocked() instead of the call
to buf_Clean().  Another approach would be registering a background
store event.  In any case, that is for another patchset.

Change-Id: I4c6d08d2a81b36f3cc43ada96acfa0ff9db23845
Reviewed-on: http://gerrit.openafs.org/6172
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>