x86: Fix non-PC platform kernel crash on boot due to NULL dereference

Upstream commit:

  95d76acc7518d5 ("x86, irq: Count legacy IRQs by legacy_pic->nr_legacy_irqs instead of NR_IRQS_LEGACY")

removed reserved interrupts for the platforms that do not have a legacy IOAPIC.

Which breaks the boot on Intel MID platforms such as Medfield:

  BUG: unable to handle kernel NULL pointer dereference at 0000003a
  IP: [<c107079a>] setup_irq+0xf/0x4d [    0.000000] *pdpt = 0000000000000000 *pde = 9bbf32453167e510

The culprit is an uncoditional setting of IRQ2 which is used
as cascade IRQ on legacy platforms. It seems we have to check
if we have enough legacy IRQs reserved before we can call
setup_irq().

The fix adds such check in native_init_IRQ() and in setup_default_timer_irq().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Jiang Liu <jiang.liu@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: David Cohen <david.a.cohen@linux.intel.com>
Link: http://lkml.kernel.org/r/1405931920-12871-1-git-send-email-andriy.shevchenko@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/irqinit.c

@@ -203,7 +203,7 @@ void __init native_init_IRQ(void)
 		set_intr_gate(i, interrupt[i - FIRST_EXTERNAL_VECTOR]);
 	}
 
-	if (!acpi_ioapic && !of_ioapic)
+	if (!acpi_ioapic && !of_ioapic && nr_legacy_irqs())
 		setup_irq(2, &irq2);
 
 #ifdef CONFIG_X86_32

arch/x86/kernel/time.c

@@ -68,6 +68,8 @@ static struct irqaction irq0  = {
 
 void __init setup_default_timer_irq(void)
 {
+	if (!nr_legacy_irqs())
+		return;
 	setup_irq(0, &irq0);
 }