sched: use TASK_NICE for task_struct

#define TASK_NICE(p)            PRIO_TO_NICE((p)->static_prio)

So it's better to use TASK_NICE here.

Signed-off-by: WANG Cong <wangcong@zeuux.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>