net_sched: fix call_rcu() race on act_sample module removal

Similar to commit c78e174
("net: sched: fix call_rcu() race on classifier module unloads"),
we need to wait for flying RCU callback tcf_sample_cleanup_rcu().

Cc: Yotam Gigi <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Jiri Pirko <[email protected]>
Cc: Jamal Hadi Salim <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/sched/act_sample.c

@@ -264,6 +264,7 @@ static int __init sample_init_module(void)
 
 static void __exit sample_cleanup_module(void)
 {
+	rcu_barrier();
 	tcf_unregister_action(&act_sample_ops, &sample_net_ops);
 }