Add generic helpers for arch IPI function calls

Add generic helpers for arch IPI function calls

This adds kernel/smp.c which contains helpers for IPI function calls. In
addition to supporting the existing smp_call_function() in a more efficient
manner, it also adds a more scalable variant called smp_call_function_single()
for calling a given function on a single CPU only.

The core of this is based on the x86-64 patch from Nick Piggin, lots of
changes since then. "Alan D. Brunelle" <[email protected]> has
contributed lots of fixes and suggestions as well. Also thanks to
Paul E. McKenney <[email protected]> for reviewing RCU usage
and getting rid of the data allocation fallback deadlock.

Acked-by: Ingo Molnar <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>