sparseirq: make for_each_irq_desc() more robust

Raja reported for_each_irq_desc() has possibility unsafeness:

if anyone write folliwing code, for_each_irq_desc() doesn't work
as intended:

(right now this code does not exist at all)

 if (safe)
   for_each_irq_desc(irq, desc) {
      ...
   }
 else
   panic();
Reported-by: NRaja R Harinath <harinath@hurrynot.org>
Signed-off-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

include/linux/irqnr.h

@@ -28,13 +28,17 @@ extern struct irq_desc *irq_to_desc(unsigned int irq);
 # define for_each_irq_desc(irq, desc)					\
 	for (irq = 0, desc = irq_to_desc(irq); irq < nr_irqs;		\
 	     irq++, desc = irq_to_desc(irq))				\
-		if (desc)
+		if (!desc)						\
+			;						\
+		else
 
 
 # define for_each_irq_desc_reverse(irq, desc)				\
 	for (irq = nr_irqs - 1, desc = irq_to_desc(irq); irq >= 0;	\
 	     irq--, desc = irq_to_desc(irq))				\
-		if (desc)
+		if (!desc)						\
+			;						\
+		else
 
 #endif /* CONFIG_GENERIC_HARDIRQS */