Windows: add 'locked' flag to cm_FSync and call when dropping write locks

Windows: add 'locked' flag to cm_FSync and call when dropping write locks

cm_FSync should be called when releasing file server write locks.
This ensures that all dirty buffers are stored before the lock
can be acquired by another client.

Since cm_Unlock holds the cm_scache_t->rw exclusively when calling
cm_FSync, add a 'locked' parameter to cm_FSync to avoid an unnecessary
release and reacquire of the rwlock.

LICENSE MIT

Change-Id: I70bbeffc24ec4238461281bab4006b3a57b275e2
Reviewed-on: http://gerrit.openafs.org/1876
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Asanka Herath <asanka@secure-endpoints.com>
Reviewed-by: Asanka Herath <asanka@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>