Windows: avoid race in cm_GetNewSCache

Windows: avoid race in cm_GetNewSCache

The cm_scacheLock is dropped while walking the scache LRU queue.
As a result it is possible for the cm_scache_t that is being
considered for recycling to be accessed and moved to the head
of the queue.

Track the prev and next pointers so it is possible to detect if
the cm_scache_t that is about to be recycled has been moved.  If
so, restart the search from the tail.

Change-Id: I6c3b645b85aa60197b9b6d60cffdcb818eb6f4b2
Reviewed-on: http://gerrit.openafs.org/6424
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>