selftest: packetdrill: Add max RTO test for SYN+ACK.

selftest: packetdrill: Add max RTO test for SYN+ACK.

This script sets net.ipv4.tcp_rto_max_ms to 1000 and checks
if SYN+ACK RTO is capped at 1s for TFO and non-TFO.

Without the previous patch, the max RTO is applied to TFO
SYN+ACK only, and non-TFO SYN+ACK RTO increases exponentially.

  # selftests: net/packetdrill: tcp_rto_synack_rto_max.pkt
  # TAP version 13
  # 1..2
  # tcp_rto_synack_rto_max.pkt:46: error handling packet: timing error:
     expected outbound packet at 5.091936 sec but happened at 6.107826 sec; tolerance 0.127974 sec
  # script packet:  5.091936 S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK>
  # actual packet:  6.107826 S. 0:0(0) ack 1 win 65535 <mss 1460,nop,nop,sackOK>
  # not ok 1 ipv4
  # tcp_rto_synack_rto_max.pkt:46: error handling packet: timing error:
     expected outbound packet at 5.075901 sec but happened at 6.091841 sec; tolerance 0.127976 sec
  # script packet:  5.075901 S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK>
  # actual packet:  6.091841 S. 0:0(0) ack 1 win 65535 <mss 1460,nop,nop,sackOK>
  # not ok 2 ipv6
  # # Totals: pass:0 fail:2 xfail:0 xpass:0 skip:0 error:0
  not ok 49 selftests: net/packetdrill: tcp_rto_synack_rto_max.pkt # exit=1

With the previous patch, all SYN+ACKs are retransmitted
after 1s.

  # selftests: net/packetdrill: tcp_rto_synack_rto_max.pkt
  # TAP version 13
  # 1..2
  # ok 1 ipv4
  # ok 2 ipv6
  # # Totals: pass:2 fail:0 xfail:0 xpass:0 skip:0 error:0
  ok 49 selftests: net/packetdrill: tcp_rto_synack_rto_max.pkt

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251106003357.273403-7-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>