Even though I'm not quite sure that the call_func stuff will work properly in all the places/cases IPI messages will be generated, at least be consistent with how the call_data pointer is assigned and cleared (ie, all done inside the spinlock.
Even though I'm not quite sure that the call_func stuff will work properly
in all the places/cases IPI messages will be generated, at least be consistent
with how the call_data pointer is assigned and cleared (ie, all done inside
the spinlock.

Ensure that its NULL before continuing, just to try and identify situations
where things are going horribly wrong.

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