tcp: Destroy TCP-AO, TCP-MD5 keys in .sk_destruct()

Currently there are a couple of minor issues with destroying the keys
tcp_v4_destroy_sock():

1. The socket is yet in TCP bind buckets, making it reachable for
incoming segments [on another CPU core], potentially available to send
late FIN/ACK/RST replies.

2. There is at least one code path, where tcp_done() is called before
sending RST [kudos to Bob for investigation]. This is a case of
a server, that finished sending its data and just called close().

The socket is in TCP_FIN_WAIT2 and has RCV_SHUTDOWN (set by
__tcp_close())

tcp_v4_do_rcv()/tcp_v6_do_rcv()
tcp_rcv_state_process() /* LINUX_MIB_TCPABORTONDATA */
tcp_reset()
tcp_done_with_error()
tcp_done()
inet_csk_destroy_sock() /* Destroys AO/MD5 keys */
/* tcp_rcv_state_process() returns SKB_DROP_REASON_TCP_ABORT_ON_DATA */
tcp_v4_send_reset() /* Sends an unsigned RST segment */

tcpdump:
> 22:53:15.399377 00:00:b2:1f:00:00 > 00:00:01:01:00:00, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 33929, offset 0, flags [DF], proto TCP (6), length 60)
> 1.0.0.1.34567 > 1.0.0.2.49848: Flags [F.], seq 2185658590, ack 3969644355, win 502, options [nop,nop,md5 valid], length 0
> 22:53:15.399396 00:00:01:01:00:00 > 00:00:b2:1f:00:00, ethertype IPv4 (0x0800), length 86: (tos 0x0, ttl 64, id 51951, offset 0, flags [DF], proto TCP (6), length 72)
> 1.0.0.2.49848 > 1.0.0.1.34567: Flags [.], seq 3969644375, ack 2185658591, win 128, options [nop,nop,md5 valid,nop,nop,sack 1 {2185658590:2185658591}], length 0
> 22:53:16.429588 00:00:b2:1f:00:00 > 00:00:01:01:00:00, ethertype IPv4 (0x0800), length 60: (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 40)
> 1.0.0.1.34567 > 1.0.0.2.49848: Flags [R], seq 2185658590, win 0, length 0
> 22:53:16.664725 00:00:b2:1f:00:00 > 00:00:01:01:00:00, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 60)
> 1.0.0.1.34567 > 1.0.0.2.49848: Flags [R], seq 2185658591, win 0, options [nop,nop,md5 valid], length 0
> 22:53:17.289832 00:00:b2:1f:00:00 > 00:00:01:01:00:00, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 60)
> 1.0.0.1.34567 > 1.0.0.2.49848: Flags [R], seq 2185658591, win 0, options [nop,nop,md5 valid], length 0

Note the signed RSTs later in the dump - those are sent by the server
when the fin-wait socket gets removed from hash buckets, by
the listener socket.

Instead of destroying AO/MD5 info and their keys in inet_csk_destroy_sock(),
slightly delay it until the actual socket .sk_destruct(). As shutdown'ed
socket can yet send non-data replies, they should be signed in order for
the peer to process them. Now it also matches how AO/MD5 gets destructed
for TIME-WAIT sockets (in tcp_twsk_destructor()).

This seems optimal for TCP-MD5, while for TCP-AO it seems to have an
open problem: once RST get sent and socket gets actually destructed,
there is no information on the initial sequence numbers. So, in case
this last RST gets lost in the network, the server's listener socket
won't be able to properly sign another RST. Nothing in RFC 1122
prescribes keeping any local state after non-graceful reset.
Luckily, BGP are known to use keep alive(s).

While the issue is quite minor/cosmetic, these days monitoring network
counters is a common practice and getting invalid signed segments from
a trusted BGP peer can get customers worried.

Investigated-by: Bob Gilligan <gilligan@arista.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://patch.msgid.link/20250909-b4-tcp-ao-md5-rst-finwait2-v5-1-9ffaaaf8b236@arista.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>