Windows: use lock conversion in cm_EndDirOp

Windows: use lock conversion in cm_EndDirOp

Instead of dropping the lock for read and reacquiring for write
use lock_ConvertRToW() which will make the change atomicly if
it is possible or place the thread into the wait list if not.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/5360
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 33af0f92e9b8a50c5a6dd17a9065c0ef58597c28)

Change-Id: Id92298fa848becb4131139579fd31b0c5f98a4ad
Reviewed-on: http://gerrit.openafs.org/6032
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>