sched_ext: Return NULL in llc_span

Use NULL instead of 0 to signal no LLC domain, matching numa_span() and
the function comment.

No functional change.

Signed-off-by: Cheng-Yang Chou <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: EricccTaiwan

kernel/sched/ext_idle.c

@@ -249,7 +249,7 @@ static struct cpumask *llc_span(s32 cpu)
 
 	sd = rcu_dereference(per_cpu(sd_llc, cpu));
 	if (!sd)
-		return 0;
+		return NULL;
 
 	return sched_domain_span(sd);
 }