Windows: Permit cm_scache rwlock to be dropped when "Stablized"

Windows: Permit cm_scache rwlock to be dropped when "Stablized"

The cm_buf_opts_t cm_BufStabilize() function was implemented
such that holding the cm_scache_t.rw lock had to be exclusively
held until cm_BufUnstablize() was called.  Unfortunately, this
prevents using Stabilize/Unstabilize to protect the cm_scache_t
during Flush operations as the cm_scache_t.rw lock must be
acquired after the cm_buf_t mutex and not before it.

This patchset reimplements the synchronization logic using
the new CM_SCACHEFLAG_SIZESETTING flag and cm_SyncOp().

LICENSE MIT

Change-Id: Iaada83f7f3b75bb3b213b33b2399e900e48a2fbc
Reviewed-on: http://gerrit.openafs.org/2661
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>