generic: sparse irqs: use irq_desc() together with dyn_array, instead of irq_desc[]

add CONFIG_HAVE_SPARSE_IRQ to for use condensed array.
Get rid of irq_desc[] array assumptions.

Preallocate 32 irq_desc, and irq_desc() will try to get more.

( No change in functionality is expected anywhere, except the odd build
  failure where we missed a code site or where a crossing commit itroduces
  new irq_desc[] usage. )

v2: according to Eric, change get_irq_desc() to irq_desc()

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/irq/spurious.c

@@ -92,11 +92,12 @@ static int misrouted_irq(int irq)
 	int ok = 0;
 
 	for (i = 1; i < nr_irqs; i++) {
-		struct irq_desc *desc = irq_desc + i;
+		struct irq_desc *desc;
 
 		if (i == irq)	/* Already tried */
 			continue;
 
+		desc = irq_to_desc(i);
 		if (try_one_irq(i, desc))
 			ok = 1;
 	}
@@ -108,7 +109,7 @@ static void poll_spurious_irqs(unsigned long dummy)
 {
 	int i;
 	for (i = 1; i < nr_irqs; i++) {
-		struct irq_desc *desc = irq_desc + i;
+		struct irq_desc *desc = irq_to_desc(i);
 		unsigned int status;
 
 		/* Racy but it doesn't matter */