sched_ext: Test sched_class directly in scx_task_iter_next_filtered()

scx_task_iter_next_filtered() is used to iterate all non-idle tasks in the
init and exit paths. Idle tasks are determined using is_idle_task().
Unfortunately, cff9b23 ("kernel/sched: Modify initial boot task idle
setup") changed idle task initialization so that %PF_IDLE is set during CPU
startup. So, CPUs that are not brought up during boot (such as CPUs which
can never be online in some AMD processors) don't have the flag set and thus
fails is_idle_task() test.

This makes sched_ext incorrectly try to operate on idle tasks in init/exit
paths leading to oopses. Fix it by directly testing p->sched_class against
idle_sched_class.

kernel/sched/ext.c

@@ -432,7 +432,11 @@ scx_task_iter_next_filtered(struct scx_task_iter *iter)
 	struct task_struct *p;
 
 	while ((p = scx_task_iter_next(iter))) {
-		if (!is_idle_task(p))
+		/*
+		 * is_idle_task() tests %PF_IDLE which may not be set for CPUs
+		 * which haven't yet been onlined. Test sched_class directly.
+		 */
+		if (p->sched_class != &idle_sched_class)
 			return p;
 	}
 	return NULL;