tcp: be more strict before accepting ECN negociation

It appears some networks play bad games with the two bits reserved for
ECN. This can trigger false congestion notifications and very slow
transferts.

Since RFC 3168 (6.1.1) forbids SYN packets to carry CT bits, we can
disable TCP ECN negociation if it happens we receive mangled CT bits in
the SYN packet.

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Perry Lorier <[email protected]>
Cc: Matt Mathis <[email protected]>
Cc: Yuchung Cheng <[email protected]>
Cc: Neal Cardwell <[email protected]>
Cc: Wilmer van der Gaast <[email protected]>
Cc: Ankur Jain <[email protected]>
Cc: Tom Herbert <[email protected]>
Cc: Dave Täht <[email protected]>
Acked-by: Neal Cardwell <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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