dctcp: more accurate tracking of packets delivery

After commit e21db6f69a95 ("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 <[email protected]>
Cc: Yuchung Cheng <[email protected]>
Cc: Neal Cardwell <[email protected]>
Cc: Soheil Hassas Yeganeh <[email protected]>
Cc: Florian Westphal <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Lawrence Brakmo <[email protected]>
Cc: Abdul Kabbani <[email protected]>
Acked-by: Soheil Hassas Yeganeh <[email protected]>
Acked-by: Neal Cardwell <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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