mptcp: fix warn at shutdown time for unaccepted msk sockets

With commit b93df08c ("mptcp: explicitly track the fully
established status"), the status of unaccepted mptcp closed in
mptcp_sock_destruct() changes from TCP_SYN_RECV to TCP_ESTABLISHED.

As a result mptcp_sock_destruct() does not perform the proper
cleanup and inet_sock_destruct() will later emit a warn.

Address the issue updating the condition tested in mptcp_sock_destruct().
Also update the related comment.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/66Reported-and-tested-by: Christoph Paasch <cpaasch@apple.com>
Fixes: b93df08c ("mptcp: explicitly track the fully established status")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/subflow.c

@@ -423,12 +423,12 @@ static void mptcp_sock_destruct(struct sock *sk)
 	 * also remove the mptcp socket, via
 	 * sock_put(ctx->conn).
 	 *
-	 * Problem is that the mptcp socket will not be in
-	 * SYN_RECV state and doesn't have SOCK_DEAD flag.
+	 * Problem is that the mptcp socket will be in
+	 * ESTABLISHED state and will not have the SOCK_DEAD flag.
 	 * Both result in warnings from inet_sock_destruct.
 	 */
 
-	if (sk->sk_state == TCP_SYN_RECV) {
+	if (sk->sk_state == TCP_ESTABLISHED) {
 		sk->sk_state = TCP_CLOSE;
 		WARN_ON_ONCE(sk->sk_socket);
 		sock_orphan(sk);