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=71721Reported-and-tested-by: NRichard Musil <richard.musil@gmail.com>
Signed-off-by: NLan Tianyu <tianyu.lan@intel.com>
Cc: 3.11+ <stable@vger.kernel.org> # 3.11+
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>