sched: Remove struct sched_class::next field

Now that the sched_class descriptors are defined in order via the linker
script vmlinux.lds.h, there's no reason to have a "next" pointer to the
previous priroity structure. The order of the sturctures can be aligned as
an array, and used to index and find the next sched_class descriptor.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191219214558.845353593@goodmis.org

kernel/sched/stop_task.c

@@ -111,7 +111,6 @@ static void update_curr_stop(struct rq *rq)
  */
 const struct sched_class stop_sched_class
 	__attribute__((section("__stop_sched_class"))) = {
-	.next			= &dl_sched_class,
 
 	.enqueue_task		= enqueue_task_stop,
 	.dequeue_task		= dequeue_task_stop,