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: fe8bd18f ("MIPS: Introduce irq_stack")
Cc: linux-mips@vger.kernel.org
Cc: stable@vger.kernel.org # v4.11+