mptcp: don't grow mptcp socket receive buffer when rcvbuf is locked

The mptcp rcvbuf size is adjusted according to the subflow rcvbuf size.
This should not be done if userspace did set a fixed value.

Fixes: 600911ff ("mptcp: add rmem queue accounting")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -141,11 +141,13 @@ static bool __mptcp_move_skbs_from_subflow(struct mptcp_sock *msk,
 	bool more_data_avail;
 	struct tcp_sock *tp;
 	bool done = false;
-	int rcvbuf;
 
-	rcvbuf = max(ssk->sk_rcvbuf, sk->sk_rcvbuf);
+	if (!(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
+		int rcvbuf = max(ssk->sk_rcvbuf, sk->sk_rcvbuf);
+
 		if (rcvbuf > sk->sk_rcvbuf)
 			sk->sk_rcvbuf = rcvbuf;
+	}
 
 	tp = tcp_sk(ssk);
 	do {