BACKPORT: tcp: new helper to calculate newly delivered

Add new helper tcp_newly_delivered() to prepare the ECN accounting change.

Change-Id: Ib0d3e922a7ebef6224c81b68a4316cc60b1b5caf
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>