tcp/dccp: fix behavior of stale SYN_RECV request sockets

When a TCP/DCCP listener is closed, its pending SYN_RECV request sockets
become stale, meaning 3WHS can not complete.

But current behavior is wrong :
incoming packets finding such stale sockets are dropped.

We need instead to cleanup the request socket and perform another
lookup :
- Incoming ACK will give a RST answer,
- SYN rtx might find another listener if available.
- We expedite cleanup of request sockets and old listener socket.

Fixes: 079096f103 ("tcp/dccp: install syn_recv requests into ehash table")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -1363,6 +1363,7 @@ static int tcp_v6_rcv(struct sk_buff *skb)
 	th = tcp_hdr(skb);
 	hdr = ipv6_hdr(skb);
 
+lookup:
 	sk = __inet6_lookup_skb(&tcp_hashinfo, skb, th->source, th->dest,
 				inet6_iif(skb));
 	if (!sk)
@@ -1382,8 +1383,12 @@ process:
 			reqsk_put(req);
 			goto discard_it;
 		}
-		if (sk->sk_state == TCP_LISTEN)
+		if (likely(sk->sk_state == TCP_LISTEN)) {
 			nsk = tcp_check_req(sk, skb, req, false);
+		} else {
+			inet_csk_reqsk_queue_drop(sk, req);
+			goto lookup;
+		}
 		if (!nsk) {
 			reqsk_put(req);
 			goto discard_it;