Windows: prevent infinite idle error response

Windows: prevent infinite idle error response

The idle error value (if any) is stored in the cm_req_t object.
Since the value is never cleared, the same value can be returned
for all requests that make use of the same cm_req_t object.

Change the behavior to only return an idle error once and then
clear it.

LICENSE MIT

Change-Id: Iac9ff8e56e856c2f0e440a34ac39ccd68de37c77
Reviewed-on: http://gerrit.openafs.org/1773
Reviewed-by: Asanka Herath <asanka@secure-endpoints.com>
Tested-by: Asanka Herath <asanka@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>