Windows: Modify signature of buf_CleanAsync and buf_CleanAsyncLocked

Windows: Modify signature of buf_CleanAsync and buf_CleanAsyncLocked

The buf_CleanAsync() and buf_CleanAsyncLocked() signature does
not include a cm_scache_t pointer even though buf_CleanAsyncLocked()
needs a pointer to the matching cm_scache_t object.  There are
some calls when the cm_scache_t object is already known.  For those
cases it is more efficient to avoid the additional lookup especially
when buf_CleanAsync*() is being called on every buffer associated
with the cm_scache_t object.

At the same time add a flags field and a constant
CM_BUF_WRITE_SCP_LOCKED to permit the lock state of the cm_scache_t
to be passed in.

Finally, fix up the usage in buf_FlushCleanPages() which gains
the most from these changes.

LICENSE MIT

Change-Id: I3726441ff83a89e24d790174ca71396d633f1be6
Reviewed-on: http://gerrit.openafs.org/2662
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>