Windows: Avoid race during PIOCtl DirNode allocation

Windows: Avoid race during PIOCtl DirNode allocation

Use InterlockedCompareExchangePointer to assign the DirNode to
ObjectInfo->Specific.Directory.PIOCtlDirectoryCB.  Otherwise,
one thread could race with another thread when allocating the
pioctl object.

Change-Id: Ic5b1a0ff2e44f2c4520cc7f5e536bd876bc83a65
Reviewed-on: http://gerrit.openafs.org/6661
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>