sched: Use clamp() and clamp_val() to make sys_nice() more readable

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1399541715-19568-1-git-send-email-yangds.fnst@cn.fujitsu.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -3057,17 +3057,10 @@ SYSCALL_DEFINE1(nice, int, increment)
 	 * We don't have to worry. Conceptually one call occurs first
 	 * and we have a single winner.
 	 */
-	if (increment < -40)
-		increment = -40;
-	if (increment > 40)
-		increment = 40;
-
+	increment = clamp(increment, -NICE_WIDTH, NICE_WIDTH);
 	nice = task_nice(current) + increment;
-	if (nice < MIN_NICE)
-		nice = MIN_NICE;
-	if (nice > MAX_NICE)
-		nice = MAX_NICE;
 
+	nice = clamp_val(nice, MIN_NICE, MAX_NICE);
 	if (increment < 0 && !can_nice(current, nice))
 		return -EPERM;