x86_64: rename irq_desc/irq_desc_alloc

change names:

          irq_desc() ==> irq_desc_alloc
	__irq_desc() ==> irq_desc

Also split a few of the uses in lowlevel x86 code.

v2: need to check if desc is null in smp_irq_move_cleanup

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

arch/x86/kernel/irqinit_64.c

@@ -143,7 +143,8 @@ void __init init_ISA_irqs(void)
 	init_8259A(0);
 
 	for (i = 0; i < 16; i++) {
-		struct irq_desc *desc = irq_to_desc(i);
+		/* first time call this irq_desc */
+		struct irq_desc *desc = irq_to_desc_alloc(i);
 
 		desc->status = IRQ_DISABLED;
 		desc->action = NULL;