Windows: prevent race assigning Fcb in AFSInitFcb()

Windows: prevent race assigning Fcb in AFSInitFcb()

AFSInitFcb() is executed when the ObjectInformation->Fcb pointer
is NULL.  More than one thread can make that determination at the
same time.  Use InterlockedCompareExchangePointer() to detect
a race and permit cleanup to be performed.

Remove the output parameter of AFSInitFcb() to avoid a double
assignment.

Change-Id: I3870cccd5cd5e95134446523cce3547a2135d5e3
Reviewed-on: http://gerrit.openafs.org/6562
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>