tcp: reduce calls to tcp_schedule_loss_probe()

tcp: reduce calls to tcp_schedule_loss_probe()

For RPC workloads, we alternate tcp_schedule_loss_probe() calls from
output path and from input path, with tp->packets_out value
oscillating between !zero and zero, leading to poor branch prediction.

Move tp->packets_out check from tcp_schedule_loss_probe() to
tcp_set_xmit_timer().

We avoid one call to tcp_schedule_loss_probe() from tcp_ack()
path for typical RPC workloads, while improving branch prediction.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260223113501.4070245-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>