ACK send time (tcp_time_stamp of SYNACK send instead of
rcv_tsecr), and (b) for TFO connections that do not have TCP
timestamps but retransmit the SYNACK we took a SYNACK RTT sample when
we should not take a sample.

This fix merely moves the snt_synack update logic a bit earlier in the
function, so that connections using TCP Fast Open will properly do
these updates when the ACK for the SYNACK arrives.

Moving this snt_synack update logic means that with TCP_DEFER_ACCEPT
enabled we do a few instructions of wasted work on each bare ACK, but
that seems OK.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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