scx: Fix error condition check in __scx_bpf_consume_task()

__scx_bpf_consume_task() should test whether the task is still on the DSQ
that's being iterated before erroring out if it's on a local DSQ. Otherwise,
another racing consumer can move the task to a local DSQ triggering a
spurious error. Relocate the error condition.

Author: htejun

kernel/sched/ext.c

@@ -6092,14 +6092,14 @@ __bpf_kfunc bool __scx_bpf_consume_task(unsigned long it, struct task_struct *p)
 	 */
 	dsq = READ_ONCE(p->scx.dsq);
 
+	if (unlikely(!dsq || dsq != kit_dsq))
+		return false;
+
 	if (unlikely(dsq->id == SCX_DSQ_LOCAL)) {
 		scx_ops_error("local DSQ not allowed");
 		return false;
 	}
 
-	if (unlikely(!dsq || dsq != kit_dsq))
-		return false;
-
 	if (!scx_kf_allowed(SCX_KF_DISPATCH))
 		return false;