sched/core: Stop using magic values in sched_dynamic_mode()

Use the enum names which are also what is used in the switch() in
sched_dynamic_update().

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: https://lore.kernel.org/r/20210325004515.531631-1-linux@rasmusvillemoes.dk

kernel/sched/core.c

@@ -5376,13 +5376,13 @@ static int preempt_dynamic_mode = preempt_dynamic_full;
 static int sched_dynamic_mode(const char *str)
 {
 	if (!strcmp(str, "none"))
-		return 0;
+		return preempt_dynamic_none;
 
 	if (!strcmp(str, "voluntary"))
-		return 1;
+		return preempt_dynamic_voluntary;
 
 	if (!strcmp(str, "full"))
-		return 2;
+		return preempt_dynamic_full;
 
 	return -1;
 }