powerpc: Use cpu_thread_in_core in smp_init for of_spin_map

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>
diff --git a/arch/powerpc/platforms/cell/smp.c b/arch/powerpc/platforms/cell/smp.c
index efb3964..c0d86e1 100644
--- a/arch/powerpc/platforms/cell/smp.c
+++ b/arch/powerpc/platforms/cell/smp.c
@@ -54,8 +54,8 @@
 #endif
 
 /*
- * The primary thread of each non-boot processor is recorded here before
- * smp init.
+ * The Primary thread of each non-boot processor was started from the OF client
+ * interface by prom_hold_cpus and is spinning on secondary_hold_spinloop.
  */
 static cpumask_t of_spin_map;
 
@@ -208,11 +208,7 @@
 	/* Mark threads which are still spinning in hold loops. */
 	if (cpu_has_feature(CPU_FTR_SMT)) {
 		for_each_present_cpu(i) {
-			if (i % 2 == 0)
-				/*
-				 * Even-numbered logical cpus correspond to
-				 * primary threads.
-				 */
+			if (cpu_thread_in_core(i) == 0)
 				cpu_set(i, of_spin_map);
 		}
 	} else {