mptcp: fix recvbuffer adjust on sleeping rcvmsg

If the recvmsg() blocks after receiving some data - i.e. due to
SO_RCVLOWAT - the MPTCP code will attempt multiple times to
adjust the receive buffer size, wrongly accounting every time the
cumulative of received data - instead of accounting only for the
delta.

Address the issue moving mptcp_rcv_space_adjust just after the
data reception and passing it only the just received bytes.

This also removes an unneeded difference between the TCP and MPTCP
RX code path implementation.

Fixes: 5813022 ("mptcp: error out earlier on disconnect")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20241230-net-mptcp-rbuf-fixes-v1-1-8608af434ceb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

net/mptcp/protocol.c

@@ -1939,6 +1939,8 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	goto out;
 }
 
+static void mptcp_rcv_space_adjust(struct mptcp_sock *msk, int copied);
+
 static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
 				struct msghdr *msg,
 				size_t len, int flags,
@@ -1992,6 +1994,7 @@ static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
 			break;
 	}
 
+	mptcp_rcv_space_adjust(msk, copied);
 	return copied;
 }
 
@@ -2268,16 +2271,13 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 		}
 
 		pr_debug("block timeout %ld\n", timeo);
-		mptcp_rcv_space_adjust(msk, copied);
 		err = sk_wait_data(sk, &timeo, NULL);
 		if (err < 0) {
 			err = copied ? : err;
 			goto out_err;
 		}
 	}
 
-	mptcp_rcv_space_adjust(msk, copied);
-
 out_err:
 	if (cmsg_flags && copied >= 0) {
 		if (cmsg_flags & MPTCP_CMSG_TS)