tcp: more accurately detect spurious TLP probes

Previously TLP is considered spurious if the sender receives any
DSACK during a TLP episode. This patch further checks the DSACK
sequences match the TLP's to improve accuracy.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>