PM / hibernate: no kernel_power_off when pm_power_off NULL

Reboot logic in kernel/reboot will avoid calling kernel_power_off
when pm_power_off is null, and instead uses kernel_halt. Change
hibernate's power_down to follow the behavior in the reboot call.

Calling the notifier twice (once for SYS_POWER_OFF and again for
SYS_HALT) causes a panic during hibernation on Kirkwood
Openblocks A6 board.

Signed-off-by: Sebastian Capella <[e-mail]>
Reported-by: Ezequiel Garcia <[e-mail]>
Reviewed-by: Pavel Machek <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

kernel/power/hibernate.c

···
 	case HIBERNATION_PLATFORM:
 		hibernation_platform_enter();
 	case HIBERNATION_SHUTDOWN:
-		kernel_power_off();
+		if (pm_power_off)
+			kernel_power_off();
 		break;
 #ifdef CONFIG_SUSPEND
 	case HIBERNATION_SUSPEND:
···
 	 * corruption after resume.
 	 */
 	printk(KERN_CRIT "PM: Please power down manually\n");
-	while(1);
+	while (1)
+		cpu_relax();
 }
 
 /**