x86, irq: don't call mp_config_acpi_gsi() if update_mptable is not enabled

Len expressed concern that the update_mptable feature has
side-effects on the ACPI code.

Make it sure explicitly that the code only ever gets called if
the (default disabled) update_mptable boot quirk option is
disabled.

[ Impact: isolate the update_mptable feature from ACPI code more ]

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Len Brown <lenb@kernel.org>
LKML-Reference: <4A0DC832.5090200@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/acpi/boot.c

@@ -1226,7 +1226,9 @@ int mp_register_gsi(struct device *dev, u32 gsi, int trigger, int polarity)
 		       ioapic_pin);
 		return gsi;
 	}
-	mp_config_acpi_gsi(dev, gsi, trigger, polarity);
+
+	if (enable_update_mptable)
+		mp_config_acpi_gsi(dev, gsi, trigger, polarity);
 
 	set_io_apic_irq_attr(&irq_attr, ioapic, ioapic_pin,
 			     trigger == ACPI_EDGE_SENSITIVE ? 0 : 1,