Windows: cm_GetNewSCache drop lock to permit change

Windows: cm_GetNewSCache drop lock to permit change

In cm_GetNewSCache the entire LRU queue is searched for a
cm_scache_t object that is safe to recycle.  If none are the LRU
queue was immediately searched again without dropping the
cm_scacheLock or taking a pause.  As a result it is quite possible
that a thread about to release a cm_scache_t was blocked from
doing so.

This patchset factors some of the logic a bit differently to
improve readability and adds new log messages to help diagnose
the cause of a problem if no cm_scache_t ever becomes available.

Change-Id: Ica6ebee0ce0456e879ae7188d9c8cdc935a92e5b
Reviewed-on: http://gerrit.openafs.org/7352
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>