mptcp: add statistics for mptcp socket in use

Do the statistics of mptcp socket in use with sock_prot_inuse_add().
Therefore, we can get the count of used mptcp socket from
/proc/net/protocols:

& cat /proc/net/protocols
protocol size sockets memory press maxhdr slab module cl co di ac io in de sh ss gs se re sp bi br ha uh gp em
MPTCPv6 2048 0 0 no 0 yes kernel y n y y y y y y y y y y n n n y y y n
MPTCP 1896 1 0 no 0 yes kernel y n y y y y y y y y y y n n n y y y n

Acked-by: Paolo Abeni <[e-mail]>
Signed-off-by: Menglong Dong <[e-mail]>
Signed-off-by: Mat Martineau <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/mptcp/protocol.c

···
 	return EPOLLIN | EPOLLRDNORM;
 }
 
+static void mptcp_listen_inuse_dec(struct sock *sk)
+{
+	if (inet_sk_state_load(sk) == TCP_LISTEN)
+		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
+}
+
 bool __mptcp_close(struct sock *sk, long timeout)
 {
 	struct mptcp_subflow_context *subflow;
···
 	sk->sk_shutdown = SHUTDOWN_MASK;
 
 	if ((1 << sk->sk_state) & (TCPF_LISTEN | TCPF_CLOSE)) {
+		mptcp_listen_inuse_dec(sk);
 		inet_sk_state_store(sk, TCP_CLOSE);
 		goto cleanup;
 	}
···
 	if (msk->fastopening)
 		return 0;
 
+	mptcp_listen_inuse_dec(sk);
 	inet_sk_state_store(sk, TCP_CLOSE);
 
 	mptcp_stop_timer(sk);
···
 
 	err = ssock->ops->listen(ssock, backlog);
 	inet_sk_state_store(sk, inet_sk_state_load(ssock->sk));
-	if (!err)
+	if (!err) {
+		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
 		mptcp_copy_inaddrs(sk, ssock->sk);
+	}
 
 	mptcp_event_pm_listener(ssock->sk, MPTCP_EVENT_LISTENER_CREATED);
 

net/mptcp/token.c

···
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
 	struct mptcp_sock *msk = mptcp_sk(subflow->conn);
 	int retries = MPTCP_TOKEN_MAX_RETRIES;
+	struct sock *sk = subflow->conn;
 	struct token_bucket *bucket;
 
 again:
···
 	__sk_nulls_add_node_rcu((struct sock *)msk, &bucket->msk_chain);
 	bucket->chain_len++;
 	spin_unlock_bh(&bucket->lock);
+	sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
 	return 0;
 }
 
···
 			struct mptcp_sock *msk)
 {
 	struct mptcp_subflow_request_sock *pos;
+	struct sock *sk = (struct sock *)msk;
 	struct token_bucket *bucket;
 
+	sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
 	bucket = token_bucket(req->token);
 	spin_lock_bh(&bucket->lock);
 
···
  */
 void mptcp_token_destroy(struct mptcp_sock *msk)
 {
+	struct sock *sk = (struct sock *)msk;
 	struct token_bucket *bucket;
 	struct mptcp_sock *pos;
 
 	if (sk_unhashed((struct sock *)msk))
 		return;
 
+	sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
 	bucket = token_bucket(msk->token);
 	spin_lock_bh(&bucket->lock);
 	pos = __token_lookup_msk(bucket, msk->token);