Don't call a NULL ack function in the generic IRQ code.

Some IRQ controllers don't need an ack function (e.g. OpenPIC on
PPC platforms) and for them we'd rather not have the overhead
of doing an indirect call to a function that does nothing.
Signed-off-by: Paul Mackerras <paulus@samba.org>

kernel/irq/handle.c

@@ -117,14 +117,16 @@ fastcall unsigned int __do_IRQ(unsigned int irq, struct pt_regs *regs)
 		/*
 		 * No locking required for CPU-local interrupts:
 		 */
-		desc->handler->ack(irq);
+		if (desc->handler->ack)
+			desc->handler->ack(irq);
 		action_ret = handle_IRQ_event(irq, regs, desc->action);
 		desc->handler->end(irq);
 		return 1;
 	}
 
 	spin_lock(&desc->lock);
-	desc->handler->ack(irq);
+	if (desc->handler->ack)
+		desc->handler->ack(irq);
 	/*
 	 * REPLAY is when Linux resends an IRQ that was dropped earlier
 	 * WAITING is used by probe to mark irqs that are being tested