mptcp: replace mptcp_disconnect with a stub

Paolo points out that mptcp_disconnect is bogus:
"lock_sock(sk);
looks suspicious (lock should be already held by the caller)
And call to: tcp_disconnect(sk, flags); too, sk is not a tcp
socket".

->disconnect() gets called from e.g. inet_stream_connect when
one tries to disassociate a connected socket again (to re-connect
without closing the socket first).
MPTCP however uses mptcp_stream_connect, not inet_stream_connect,
for the mptcp-socket connect call.

inet_stream_connect only gets called indirectly, for the tcp socket,
so any ->disconnect() calls end up calling tcp_disconnect for that
tcp subflow sk.

This also explains why syzkaller has not yet reported a problem
here.  So for now replace this with a stub that doesn't do anything.

Closes: multipath-tcp/mptcp_net-next#14
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -1316,11 +1316,12 @@ static void mptcp_copy_inaddrs(struct sock *msk, const struct sock *ssk)
 
 static int mptcp_disconnect(struct sock *sk, int flags)
 {
-	lock_sock(sk);
-	__mptcp_clear_xmit(sk);
-	release_sock(sk);
-	mptcp_cancel_work(sk);
-	return tcp_disconnect(sk, flags);
+	/* Should never be called.
+	 * inet_stream_connect() calls ->disconnect, but that
+	 * refers to the subflow socket, not the mptcp one.
+	 */
+	WARN_ON_ONCE(1);
+	return 0;
 }
 
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)