MIPS: irq: Allocate accurate order pages for irq stack

The irq_pages is the number of pages for irq stack, but not the
order which is needed by __get_free_pages().
We can use get_order() to calculate the accurate order.

Signed-off-by: Liu Xiang <liu.xiang6@zte.com.cn>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: fe8bd18 ("MIPS: Introduce irq_stack")
Cc: linux-mips@vger.kernel.org
Cc: stable@vger.kernel.org # v4.11+

Author: Liu Xiang

arch/mips/kernel/irq.c

@@ -52,6 +52,7 @@ asmlinkage void spurious_interrupt(void)
 void __init init_IRQ(void)
 {
 	int i;
+	unsigned int order = get_order(IRQ_STACK_SIZE);
 
 	for (i = 0; i < NR_IRQS; i++)
 		irq_set_noprobe(i);
@@ -62,8 +63,7 @@ void __init init_IRQ(void)
 	arch_init_irq();
 
 	for_each_possible_cpu(i) {
-		int irq_pages = IRQ_STACK_SIZE / PAGE_SIZE;
-		void *s = (void *)__get_free_pages(GFP_KERNEL, irq_pages);
+		void *s = (void *)__get_free_pages(GFP_KERNEL, order);
 
 		irq_stack[i] = s;
 		pr_debug("CPU%d IRQ stack at 0x%p - 0x%p\n", i,