Windows: AFSDeleteDirEntry set input to NULL

Windows: AFSDeleteDirEntry set input to NULL

AFSDeleteDirEntry() frees the memory allocated to the DirectoryCB.
To ensure that an invalid memory pointer is not accidentally used
by the caller after the memory is freed, use
InterlockedCompareExchangePointer() to set the input parameter to
NULL prior to destroying the DirectoryCB.

Change-Id: I2e92d4277d1f9baee164bfb941821aa11a1ad738
Reviewed-on: http://gerrit.openafs.org/9721
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>