Windows: AFSTearDownExtents may experience active extents

Windows: AFSTearDownExtents may experience active extents

If there are extents with a non-zero ActiveCount when AFSTearDownExtents()
is executed, it must leave them alone and attached to the File Control
Block.  This has implications for its callers, especially AFSCleanupFcb()
since it may be the case that a Cleanup cannot be completed.

The AFSPrimaryVolumeWorker thread must therefore check after calling
AFSCleanupFcb() whether or not the Fcb ExtentCount is zero before
calling AFSRemoveFcb().

Change-Id: I164dbe24d2bfe69aba0fcb5d845f66415d5bb0c3
Reviewed-on: http://gerrit.openafs.org/7406
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>