tcp: accecn: AccECN needs to know delivered bytes

tcp: accecn: AccECN needs to know delivered bytes

AccECN byte counter estimation requires delivered bytes
which can be calculated while processing SACK blocks and
cumulative ACK. The delivered bytes will be used to estimate
the byte counters between AccECN option (on ACKs w/o the
option).

Accurate ECN does not depend on SACK to function; however,
the calculation would be more accurate if SACK were there.

Signed-off-by: Ilpo Järvinen <ij@kernel.org>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250916082434.100722-5-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>