tcp: Don't flag tcp_sk(sk)->rx_opt.saw_unknown for TCP AO.

When we process segments with TCP AO, we don't check it in
tcp_parse_options().  Thus, opt_rx->saw_unknown is set to 1,
which unconditionally triggers the BPF TCP option parser.

Let's avoid the unnecessary BPF invocation.

Fixes: 0a3a809 ("net/tcp: Verify inbound TCP-AO signed segments")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20240703033508.6321-1-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: q2ven

net/ipv4/tcp_input.c

@@ -4223,6 +4223,13 @@ void tcp_parse_options(const struct net *net,
 				 * checked (see tcp_v{4,6}_rcv()).
 				 */
 				break;
+#endif
+#ifdef CONFIG_TCP_AO
+			case TCPOPT_AO:
+				/* TCP AO has already been checked
+				 * (see tcp_inbound_ao_hash()).
+				 */
+				break;
 #endif
 			case TCPOPT_FASTOPEN:
 				tcp_parse_fastopen_option(