powerpc/mpic: Break cpumask abstraction earlier

mpic_set_affinity is allocating and freeing a cpumask var even though
it was breaking the cpumask abstraction when passing the mask to
mpic_physmask.  It also didn't have any check for allocatin failure.

Break the cpumask abstraction earlier and use simple bitwise and of the
bits from the mask with the bits of cpu_online_mask.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>