stop_machine enhancements and simplifications

Hi all,

   Here are the three stop_machine patches I've queued for the next merge
window.  The first two are pretty well tested via linux-next, the last is
recent but fairly straightforward.

   I'm assuming that Jason and/or Mathieu have need for the ALL_CPUS mod,
but it can be removed by the last of these patches (left in to reduce
transition breakage).

Feedback welcome as always,
Rusty.