ACPI / button: Add ACPI Button event via netlink routine

commit 0bf6368e upstream.

Commit 1696d9dc (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>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>