sched_ext: Sync error_irq_work before freeing scx_sched

By the time scx_sched_free_rcu_work() runs, the scx_sched is no longer
reachable. However, a previously queued error_irq_work may still be pending or
running. Ensure it completes before proceeding with teardown.

Fixes: bff3b5a ("sched_ext: Move disable machinery into scx_sched")
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: htejun

kernel/sched/ext.c

@@ -3471,7 +3471,9 @@ static void scx_sched_free_rcu_work(struct work_struct *work)
 	struct scx_dispatch_q *dsq;
 	int node;
 
+	irq_work_sync(&sch->error_irq_work);
 	kthread_stop(sch->helper->task);
+
 	free_percpu(sch->pcpu);
 
 	for_each_node_state(node, N_POSSIBLE)