tcp: better handle TCP_TX_DELAY on established flows

tcp: better handle TCP_TX_DELAY on established flows

Some applications uses TCP_TX_DELAY socket option after TCP flow
is established.

Some metrics need to be updated, otherwise TCP might take time to
adapt to the new (emulated) RTT.

This patch adjusts tp->srtt_us, tp->rtt_min, icsk_rto
and sk->sk_pacing_rate.

This is best effort, and for instance icsk_rto is reset
without taking backoff into account.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251013145926.833198-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>