packet: free packet_rollover after synchronize_net

Destruction of the po->rollover must be delayed until there are no
more packets in flight that can access it. The field is destroyed in
packet_release, before synchronize_net. Delay using rcu.

Fixes: 0648ab70 ("packet: rollover prepare: per-socket state")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -1634,7 +1634,8 @@ static void fanout_release(struct sock *sk)
 	}
 	mutex_unlock(&fanout_mutex);
 
-	kfree(po->rollover);
+	if (po->rollover)
+		kfree_rcu(po->rollover, rcu);
 }
 
 static const struct proto_ops packet_ops;

net/packet/internal.h

@@ -89,6 +89,7 @@ struct packet_fanout {
 
 struct packet_rollover {
 	int			sock;
+	struct rcu_head		rcu;
 	atomic_long_t		num;
 	atomic_long_t		num_huge;
 	atomic_long_t		num_failed;