tcp: always use tcp_limit_output_bytes limitation

This partially reverts commit c73e580 ("tcp: tsq: no longer use
limit_output_bytes for paced flows")

Overriding the tcp_limit_output_bytes sysctl value
for FQ enabled flows has the following problem:

It allows TCP to queue around 2 ms worth of data per flow,
defeating tcp_rcv_rtt_update() accuracy on the receiver,
forcing it to increase sk->sk_rcvbuf even if the real
RTT is around 100 us.

After this change, we keep enough packets in flight to fill
the pipe, and let receive queues small enough to get
good cache behavior (cpu caches and/or NIC driver page pools).

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250513193919.1089692-11-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv4/tcp_output.c

@@ -2619,9 +2619,8 @@ static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
 	limit = max_t(unsigned long,
 		      2 * skb->truesize,
 		      READ_ONCE(sk->sk_pacing_rate) >> READ_ONCE(sk->sk_pacing_shift));
-	if (sk->sk_pacing_status == SK_PACING_NONE)
-		limit = min_t(unsigned long, limit,
-			      READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes));
+	limit = min_t(unsigned long, limit,
+		      READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes));
 	limit <<= factor;
 
 	if (static_branch_unlikely(&tcp_tx_delay_enabled) &&