tcp: always seek for minimal rtt in tcp_rcv_rtt_update()

tcp: always seek for minimal rtt in tcp_rcv_rtt_update()

tcp_rcv_rtt_update() goal is to maintain an estimation of the RTT
in tp->rcv_rtt_est.rtt_us, used by tcp_rcv_space_adjust()

When TCP TS are enabled, tcp_rcv_rtt_update() is using
EWMA to smooth the samples.

Change this to immediately latch the incoming value if it
is lower than tp->rcv_rtt_est.rtt_us, so that tcp_rcv_space_adjust()
does not overshoot tp->rcvq_space.space and sk->sk_rcvbuf.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250513193919.1089692-8-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>