ipv6: tcp: send consistent autoflowlabel in SYN_RECV state

ipv6: tcp: send consistent autoflowlabel in SYN_RECV state

This is a followup of commit f04eef622bc3 ("ipv6: tcp: send consistent
autoflowlabel in TIME_WAIT state"), but for SYN_RECV state.

In some cases, TCP sends a challenge ACK on behalf of a SYN_RECV request.
WHen this happens, we want to use the flow label that was used when
the prior SYNACK packet was sent, instead of another one.

After his patch, following packetdrill passes:

    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
   +0 bind(3, ..., ...) = 0
   +0 listen(3, 1) = 0

  +.2 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
   +0 > (flowlabel 0x11) S. 0:0(0) ack 1 <...>
// Test if a challenge ack is properly sent (same flowlabel than prior SYNACK)
   +.01 < . 4000000000:4000000000(0) ack 1 win 320
   +0  > (flowlabel 0x11) . 1:1(0) ack 1

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220831203729.458000-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>