Windows: AFSRemoveFcb() cannot race

Windows: AFSRemoveFcb() cannot race

Modify AFSRemoveFcb to use InterlockedComparePointerExchange
to ensure that only one thread can remove and deallocate an
AFSFcb structure.

Change-Id: I27d27b6a99806bee2fc2cfc04c2ac04d975a553d
Reviewed-on: http://gerrit.openafs.org/6696
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>