tcp: timestamp SYN+DATA messages

All skb in socket write queue should be properly timestamped.

In case of FastOpen, we special case the SYN+DATA 'message' as we
queue in socket wrote queue the two fallback skbs:

1) SYN message by itself.
2) DATA segment by itself.

We should make sure these skbs have proper timestamps.

Add a WARN_ON_ONCE() to eventually catch future violations.

Fixes: 740b0f18 ("tcp: switch rtt estimations to usec resolution")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>