powerpc/mm: Fix switch_mmu_context to iterate of the proper list of cpus

Introduced a temporary variable into our iterating over the list cpus
that are threads on the same core.  For some reason Ben forgot how for
loops work.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>