mptcp: Always call sk_data_ready

Since commit 03f45c8 ("tcp: avoid extra wakeups for SO_RCVLOWAT users"),
tcp_data_queue checks for available bytes in the subflow's receive-queue
before waking up the "app". However, with MPTCP we might have
zero-length data-fins in the queue. Thus, we still need to wakeup the
MPTCP-layer in case of a subflow.

Fixes: dd64d1c ("Merge tag 'v4.18' into mptcp_trunk")
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

net/ipv4/tcp_input.c

@@ -4736,7 +4736,7 @@ void tcp_data_ready(struct sock *sk)
 	const struct tcp_sock *tp = tcp_sk(sk);
 	int avail = tp->rcv_nxt - tp->copied_seq;
 
-	if (avail < sk->sk_rcvlowat && !sock_flag(sk, SOCK_DONE))
+	if (avail < sk->sk_rcvlowat && !sock_flag(sk, SOCK_DONE) && !mptcp(tp))
 		return;
 
 	sk->sk_data_ready(sk);