ACPI / button: Add ACPI Button event via netlink routine

Commit 1696d9d (ACPI: Remove the old /proc/acpi/event interface)
removed ACPI Button event which originally was sent to userspace via
/proc/acpi/event. This caused ACPI shutdown regression on gentoo
in VirtualBox. Now ACPI events are sent to userspace via netlink,
so add ACPI Button event back via netlink routine.

References: https://bugzilla.kernel.org/show_bug.cgi?id=71721
Reported-and-tested-by: Richard Musil <richard.musil@gmail.com>
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Cc: 3.11+ <stable@vger.kernel.org> # 3.11+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/button.c

@@ -302,6 +302,10 @@ static void acpi_button_notify(struct acpi_device *device, u32 event)
 			input_sync(input);
 
 			pm_wakeup_event(&device->dev, 0);
+			acpi_bus_generate_netlink_event(
+					device->pnp.device_class,
+					dev_name(&device->dev),
+					event, ++button->pushed);
 		}
 		break;
 	default: