sched: Revert 738d2be, simplify set_task_cpu()

Effectively reverts 738d2be4301007f054541c5c4bf7fb6a361c9b3a.

As demonstrated by Eric, we really need to call __set_task_cpu()
early in the fork() path to properly initialize the various task
state -- specifically the cgroup state through set_task_rq().

[ we could probably fix this by explicitly calling
  __set_task_cpu() from   sched_fork(), but lets try that for the
  next cycle and simply revert to the old behaviour for now. ]

Reported-by: Eric Paris <eparis@redhat.com>
Tested-by: Eric Paris <eparis@redhat.com>,
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: efault@gmx.de
LKML-Reference: <1261492999.4937.36.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -2045,11 +2045,10 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 
 	trace_sched_migrate_task(p, new_cpu);
 
-	if (task_cpu(p) == new_cpu)
-		return;
-
-	p->se.nr_migrations++;
-	perf_sw_event(PERF_COUNT_SW_CPU_MIGRATIONS, 1, 1, NULL, 0);
+	if (task_cpu(p) != new_cpu) {
+		p->se.nr_migrations++;
+		perf_sw_event(PERF_COUNT_SW_CPU_MIGRATIONS, 1, 1, NULL, 0);
+	}
 
 	__set_task_cpu(p, new_cpu);
 }