cpu_down: move migrate_enable() back

Commit 08c1ab6, "hotplug-use-migrate-disable.patch", intends to
use migrate_enable()/migrate_disable() to replace that combination
of preempt_enable() and preempt_disable(), but actually in
!CONFIG_PREEMPT_RT_FULL case, migrate_enable()/migrate_disable()
are still equal to preempt_enable()/preempt_disable(). So that
followed cpu_hotplug_begin()/cpu_unplug_begin(cpu) would go schedule()
to trigger schedule_debug() like this:

_cpu_down()
	|
	+ migrate_disable() = preempt_disable()
	|
	+ cpu_hotplug_begin() or cpu_unplug_begin()
		|
		+ schedule()
			|
			+ __schedule()
				|
				+ preempt_disable();
				|
				+ __schedule_bug() is true!

So we should move migrate_enable() as the original scheme.

Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>

kernel/cpu.c

@@ -1310,6 +1310,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
 		goto restore_cpus;
 	}
 
+	migrate_enable();
 	cpu_hotplug_begin();
 	ret = cpu_unplug_begin(cpu);
 	if (ret) {
@@ -1357,7 +1358,6 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
 	cpu_unplug_done(cpu);
 out_cancel:
 	cpu_hotplug_done();
-	migrate_enable();
 	/* This post dead nonsense must die */
 	if (!ret && hasdied)
 		cpu_notify_nofail(CPU_POST_DEAD, cpu);