powerpc: Use cpu_thread_in_core in smp_init for of_spin_map

powerpc: Use cpu_thread_in_core in smp_init for of_spin_map

commit 6a75a6b8e85e92cc774d42a4e113c76c30b5a539 upstream.

We used to assume that even numbered threads were the primary
threads, ie those that would be listed and started as a cpu from
open firmware.  Replace a left over is even (% 2) check with a check
for it being a primary thread and update the comments.

Tested with a debug print on pseries, identical code found for cell.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>