sched: Fix a stale comment in pick_next_task()

fair_sched_class->next no longer exists since commit:

  a87e749e ("sched: Remove struct sched_class::next field").

Now the sched_class order is specified by the linker script.

Rewrite the comment in a more generic way.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210519063709.323162-1-masahiroy@kernel.org

kernel/sched/core.c

@@ -5318,7 +5318,7 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 		if (unlikely(p == RETRY_TASK))
 			goto restart;
 
-		/* Assumes fair_sched_class->next == idle_sched_class */
+		/* Assume the next prioritized class is idle_sched_class */
 		if (!p) {
 			put_prev_task(rq, prev);
 			p = pick_next_task_idle(rq);