Revert "sched/core: Do not use smp_processor_id() with preempt enabled in smpboot_thread_fn()"

This reverts commit 4fa5cd52.

The original change widens a preempt-off section, to avoid a seemingly unsafe
smp_processor_id() use.

During review I overlooked two facts:

 - The code to calls a non-trivial function callback:

                                ht->park(td->cpu);

   ... which might (and does occasionally) sleep, triggering the warning.

 - More importantly, as pointed out by Peter Zijlstra, using
   smp_processor_id() in that context is safe, if it's done from
   a kernel thread that is pinned to a single CPU - which is the
   case here.

So revert to the original code that enables preemption sooner.
Reported-by: kernel test robot <xiaolong.ye@intel.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Con Kolivas <kernel@kolivas.org>
Cc: Alfred Chen <cchalpha@gmail.com>
Link: http://lkml.kernel.org/r/20160930015102.GB20189@yexl-desktopSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/smpboot.c

@@ -122,12 +122,12 @@ static int smpboot_thread_fn(void *data)
 
 		if (kthread_should_park()) {
 			__set_current_state(TASK_RUNNING);
+			preempt_enable();
 			if (ht->park && td->status == HP_THREAD_ACTIVE) {
 				BUG_ON(td->cpu != smp_processor_id());
 				ht->park(td->cpu);
 				td->status = HP_THREAD_PARKED;
 			}
-			preempt_enable();
 			kthread_parkme();
 			/* We might have been woken for stop */
 			continue;