mptcp: reset 'first' and ack_hint on subflow close

Just like with last_snd, we have to NULL 'first' on subflow close.

ack_hint isn't strictly required (its never dereferenced), but better to
clear this explicitly as well instead of making it an exception.

msk->first is dereferenced unconditionally at accept time, but
at that point the ssk is not on the conn_list yet -- this means
worker can't see it when iterating the conn_list.

Reported-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/mptcp/protocol.c

@@ -2162,6 +2162,12 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 	if (ssk == msk->last_snd)
 		msk->last_snd = NULL;
 
+	if (ssk == msk->ack_hint)
+		msk->ack_hint = NULL;
+
+	if (ssk == msk->first)
+		msk->first = NULL;
+
 	if (msk->subflow && ssk == msk->subflow->sk)
 		mptcp_dispose_initial_subflow(msk);
 }
@@ -3293,6 +3299,9 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 		/* PM/worker can now acquire the first subflow socket
 		 * lock without racing with listener queue cleanup,
 		 * we can notify it, if needed.
+		 *
+		 * Even if remote has reset the initial subflow by now
+		 * the refcnt is still at least one.
 		 */
 		subflow = mptcp_subflow_ctx(msk->first);
 		list_add(&subflow->node, &msk->conn_list);