sched_ext: Drop spurious warning on kick during scheduler disable

kick_cpus_irq_workfn() warns when scx_kick_syncs is NULL, but this can
legitimately happen when a BPF timer or other kick source races with
free_kick_syncs() during scheduler disable. Drop the pr_warn_once() and
add a comment explaining the race.

Signed-off-by: Tejun Heo <[email protected]>
Reviewed-by: Zhao Mengmeng <[email protected]>

Author: htejun

kernel/sched/ext.c

@@ -7600,10 +7600,9 @@ static void kick_cpus_irq_workfn(struct irq_work *irq_work)
 	unsigned long *ksyncs;
 	s32 cpu;
 
-	if (unlikely(!ksyncs_pcpu)) {
-		pr_warn_once("kick_cpus_irq_workfn() called with NULL scx_kick_syncs");
+	/* can race with free_kick_syncs() during scheduler disable */
+	if (unlikely(!ksyncs_pcpu))
 		return;
-	}
 
 	ksyncs = rcu_dereference_bh(ksyncs_pcpu)->syncs;