tcp/dccp: allow a connection when sk_max_ack_backlog is zero

[ Upstream commit 3479c7549fb1dfa7a1db4efb7347c7b8ef50de4b ]

If the backlog of listen() is set to zero, sk_acceptq_is_full() allows
one connection to be made, but inet_csk_reqsk_queue_is_full() does not.
When the net.ipv4.tcp_syncookies is zero, inet_csk_reqsk_queue_is_full()
will cause an immediate drop before the sk_acceptq_is_full() check in
tcp_conn_request(), resulting in no connection can be made.

This patch tries to keep consistent with 64a14651 ("[NET]: Revert
incorrect accept queue backlog changes.").

Link: https://lore.kernel.org/netdev/20250102080258.53858-1-kuniyu@amazon.com/


Fixes: ef547f2a ("tcp: remove max_qlen_log")
Signed-off-by: Zhongqiu Duan <dzq.aishenghu0@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250102171426.915276-1-dzq.aishenghu0@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>