tcp: more accurately check DSACKs to grow RACK reordering window

Previously, a DSACK could expand the RACK reordering window when no
reordering has been seen, and/or when the DSACK was due to an
unnecessary TLP retransmit (rather than a spurious fast recovery due
to reordering). This could result in unnecessarily growing the RACK
reordering window and thus unnecessarily delaying RACK-based fast
recovery episodes.

To avoid these issues, this commit tightens the conditions under which
a DSACK triggers the RACK reordering window to grow, so that a
connection only expands its RACK reordering window if:

(a) reordering has been seen in the connection
(b) a DSACKed range does not match the most recent TLP retransmit

Signed-off-by: Neal Cardwell <[email protected]>
Acked-by: Yuchung Cheng <[email protected]>
Acked-by: Priyaranjan Jha <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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