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: Raja R Harinath <harinath@hurrynot.org>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>