tcp: exclude old ACKs from tcp fast path

Exclude old ACKs before SND.UNA from the tcp fast path
as well as ACKs after SND.NXT.

Such ACKs will fall through to the slow path, where tcp_ack()
performs the appropriate validation and challenge ACK handling
according to RFC5961 and Commit 3d501dd326fb1c7 ("tcp: do not
accept ACK of bytes we never sent").

This prevents old ACKs from being accepted
or modifying connection state as part of the fast path before
appropriate ACK validation is applied.
In particular, this prevents payload carried by a segment with
an excessively old ACK from advancing RCV.NXT before the ACK
is rejected.

Fixes: 31770e34e43d ("tcp: Revert "tcp: remove header prediction"")
Reported-by: Amit Klein <amit.klein@mail.huji.ac.il>
Reported-by: Tamir Shahar <tamir.shahar1@mail.huji.ac.il>
Reported-by: Inbal Schussheim <inbal.lipshtat@mail.huji.ac.il>
Suggested-by: Eric Dumazet <edumazet@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Inbal Schussheim <inbal.lipshtat@mail.huji.ac.il>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260914090408.1435080-2-inbal.lipshtat@mail.huji.ac.il
Signed-off-by: Paolo Abeni <pabeni@redhat.com>