x86/ioapic: Prevent NULL pointer dereference in setup_ioapic_dest()

Commit 4857c91f0d changed the way how irq affinity is setup in
setup_ioapic_dest() from using the core helper function to
unconditionally calling the irq_set_affinity() callback of the
underlying irq chip.

That results in a NULL pointer dereference for the rare case where the
underlying irq chip is lapic_chip which has no irq_set_affinity()
callback. lapic_chip is occasionally used for the timer interrupt (irq
0).

The fix is simple: Check the availability of the callback instead of
calling it unconditionally.

Fixes: 4857c91f0d "x86/ioapic: Force affinity setting in setup_ioapic_dest()"
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org

arch/x86/kernel/apic/io_apic.c

@@ -2547,6 +2547,8 @@ void __init setup_ioapic_dest(void)
 			mask = apic->target_cpus();
 
 		chip = irq_data_get_irq_chip(idata);
+		/* Might be lapic_chip for irq 0 */
+		if (chip->irq_set_affinity)
 			chip->irq_set_affinity(idata, mask, false);
 	}
 }