x86, io-apic: fix nmi_watchdog=1 bootup hang

nmi_watchdog=1 hangs on 64-bit:

[    0.250000] Detected 12.564 MHz APIC timer.
[    0.254178] APIC timer registered as dummy, due to nmi_watchdog=1!
[    0.260366] Testing NMI watchdog ... <4>WARNING: CPU#0: NMI appears to be stuck (0->0)!
[    ...     ]
[    0.470003] calling  genl_init+0x0/0xd0
[  hard hang ]

bisected it down to:

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 git-bisect bad 11582ece0aaa2d0f94f345c08a4ab9997078a083
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 git-bisect good 90e23b13ab849e2a11f00c655eb3a2011b4623be
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 git-bisect bad 317e1f2597ffb4d4db940577bbe56dc6e881ef07

| 317e1f2597ffb4d4db940577bbe56dc6e881ef07 is first bad commit
| commit 317e1f2597ffb4d4db940577bbe56dc6e881ef07
| Author: Maciej W. Rozycki <macro@linux-mips.org>
| Date:   Wed May 21 22:10:22 2008 +0100
|     x86: I/O APIC: clean up the 8259A on a NMI watchdog failure

the problem is that in the dummy-lapic branch we rely on the i8259A
but if the NMI watchdog fails we turn off IRQ 0 - which doesnt work
too well ;-)
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/apic_32.c

@@ -541,11 +541,13 @@ void __init setup_boot_APIC_clock(void)
 		 * PIT/HPET going.  Otherwise register lapic as a dummy
 		 * device.
 		 */
-		if (nmi_watchdog != NMI_IO_APIC)
+		if (nmi_watchdog != NMI_IO_APIC) {
 			lapic_clockevent.features &= ~CLOCK_EVT_FEAT_DUMMY;
-		else
+		} else {
 			printk(KERN_WARNING "APIC timer registered as dummy,"
 			       " due to nmi_watchdog=1!\n");
+			timer_through_8259 = 1;
+		}
 	}
 
 	/* Setup the lapic or request the broadcast */

arch/x86/kernel/apic_64.c

@@ -413,11 +413,13 @@ void __init setup_boot_APIC_clock(void)
 	 * PIT/HPET going.  Otherwise register lapic as a dummy
 	 * device.
 	 */
-	if (nmi_watchdog != NMI_IO_APIC)
+	if (nmi_watchdog != NMI_IO_APIC) {
 		lapic_clockevent.features &= ~CLOCK_EVT_FEAT_DUMMY;
-	else
+	} else {
 		printk(KERN_WARNING "APIC timer registered as dummy,"
 		       " due to nmi_watchdog=1!\n");
+		timer_through_8259 = 1;
+	}
 
 	setup_APIC_timer();
 }