HOTPLUG: Adapt cpuset hotplug callback to CPU_DYING

CPU_DYING is called in atomic context, so don't try to take any locks.
Signed-off-by: Avi Kivity <avi@qumranet.com>

kernel/cpuset.c

@@ -2138,6 +2138,9 @@ static void common_cpu_mem_hotplug_unplug(void)
 static int cpuset_handle_cpuhp(struct notifier_block *nb,
 				unsigned long phase, void *cpu)
 {
+	if (phase == CPU_DYING || phase == CPU_DYING_FROZEN)
+		return NOTIFY_DONE;
+
 	common_cpu_mem_hotplug_unplug();
 	return 0;
 }