tcp: refine skb->ooo_okay setting

tcp: refine skb->ooo_okay setting

Enabling BIG TCP on a low end platform apparently increased
chances of getting flows locked on one busy TX queue.

A similar problem was handled in commit 9b462d02d6dd
("tcp: TCP Small Queues and strange attractors"),
but the strategy worked for either bulk flows,
or 'large enough' RPC. BIG TCP changed how large
RPC needed to be to enable the work around:
If RPC fits in a single skb, TSQ never triggers.

Root cause for the problem is a busy TX queue,
with delayed TX completions.

This patch changes how we set skb->ooo_okay to detect
the case TX completion was not done, but incoming ACK
already was processed and emptied rtx queue.

Update the comment to explain the tricky details.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230817182353.2523746-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>