Fix SMP poweroff hangs

We need to disable all CPUs other than the boot CPU (usually 0) before
attempting to power-off modern SMP machines. This fixes the
hang-on-poweroff issue on my MythTV SMP box, and also on Thomas Gleixner's
new toybox.

Signed-off-by: Mark Lord <[e-mail]>
Acked-by: Thomas Gleixner <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/sys.c

···
 #include <linux/getcpu.h>
 #include <linux/task_io_accounting_ops.h>
 #include <linux/seccomp.h>
+#include <linux/cpu.h>
 
 #include <linux/compat.h>
 #include <linux/syscalls.h>
···
 	kernel_shutdown_prepare(SYSTEM_POWER_OFF);
 	if (pm_power_off_prepare)
 		pm_power_off_prepare();
+	disable_nonboot_cpus();
 	sysdev_shutdown();
 	printk(KERN_EMERG "Power down.\n");
 	machine_power_off();