dctcp: more accurate tracking of packets delivery

After commit e21db6f6 ("tcp: track total bytes delivered with ECN CE marks")
core TCP stack does a very good job tracking ECN signals.

The "sender's best estimate of CE information" Yuchung mentioned in his
patch is indeed the best we can do.

DCTCP can use tp->delivered_ce and tp->delivered to not duplicate the logic,
and use the existing best estimate.

This solves some problems, since current DCTCP logic does not deal with losses
and/or GRO or ack aggregation very well.

This also removes a dubious use of inet_csk(sk)->icsk_ack.rcv_mss
(this should have been tp->mss_cache), and a 64 bit divide.

Finally, we can see that the DCTCP logic, calling dctcp_update_alpha() for
every ACK could be done differently, calling it only once per RTT.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Lawrence Brakmo <brakmo@fb.com>
Cc: Abdul Kabbani <akabbani@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>