disable_irq() should synchronize with irq handler only if one exists.

Noted by Al Viro: if no handler exists (and we have IRQ_INPROGRESS set
because of an earlier irq that got through), synchronize_irq() will
end up waiting forever.

arch/i386/kernel/irq.c

@@ -355,7 +355,9 @@ inline void disable_irq_nosync(unsigned int irq)
  
 void disable_irq(unsigned int irq)
 {
+	irq_desc_t *desc = irq_desc + irq;
 	disable_irq_nosync(irq);
+	if (desc->action)
 		synchronize_irq(irq);
 }