tcp: accecn: try to fit AccECN option with SACK

As SACK blocks tend to eat all option space when there are
many holes, it is useful to compromise on sending many SACK
blocks in every ACK and attempt to fit the AccECN option
there by reducing the number of SACK blocks. However, it will
never go below two SACK blocks because of the AccECN option.

As the AccECN option is often not put to every ACK, the space
hijack is usually only temporary. Depending on the reuqired
AccECN fields (can be either 3, 2, 1, or 0, cf. Table 5 in
AccECN spec) and the NOPs used for alignment of other
TCP options, up to two SACK blocks will be reduced. Please
find below tables for more details:

+====================+=========================================+
| Number of | Required | Remaining |  Number of  |    Final    |
|   SACK    |  AccECN  |  option   |  reduced    |  number of  |
|  blocks   |  fields  |  spaces   | SACK blocks | SACK blocks |
+===========+==========+===========+=============+=============+
|  x (<=2)  |  0 to 3  |    any    |      0      |      x      |
+-----------+----------+-----------+-------------+-------------+
|     3     |    0     |    any    |      0      |      3      |
|     3     |    1     |    <4     |      1      |      2      |
|     3     |    1     |    >=4    |      0      |      3      |
|     3     |    2     |    <8     |      1      |      2      |
|     3     |    2     |    >=8    |      0      |      3      |
|     3     |    3     |    <12    |      1      |      2      |
|     3     |    3     |    >=12   |      0      |      3      |
+-----------+----------+-----------+-------------+-------------+
|  y (>=4)  |    0     |    any    |      0      |      y      |
|  y (>=4)  |    1     |    <4     |      1      |     y-1     |
|  y (>=4)  |    1     |    >=4    |      0      |      y      |
|  y (>=4)  |    2     |    <8     |      1      |     y-1     |
|  y (>=4)  |    2     |    >=8    |      0      |      y      |
|  y (>=4)  |    3     |    <4     |      2      |     y-2     |
|  y (>=4)  |    3     |    <12    |      1      |     y-1     |
|  y (>=4)  |    3     |    >=12   |      0      |      y      |
+===========+==========+===========+=============+=============+

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Co-developed-by: Ilpo Järvinen <ij@kernel.org>
Signed-off-by: Ilpo Järvinen <ij@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250916082434.100722-11-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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