The TCP_USER_TIMEOUT is checked by the 0-window probe timer. As the
timer has backoff with a max interval of about two minutes, the
actual timeout for TCP_USER_TIMEOUT can be off by up to two minutes.

In this patch the TCP_USER_TIMEOUT is made more accurate by taking it
into account when computing the timer value for the 0-window probes.

This patch is similar to and builds on top of the one that made
TCP_USER_TIMEOUT accurate for RTOs in commit b701a99e ("tcp: Add
tcp_clamp_rto_to_user_timeout() helper to improve accuracy").

Fixes: 9721e709 ("tcp: simplify window probe aborting on USER_TIMEOUT")
Signed-off-by: NEnke Chen <enchen@paloaltonetworks.com>
Reviewed-by: NNeal Cardwell <ncardwell@google.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20210122191306.GA99540@localhost.localdomainSigned-off-by: NJakub Kicinski <kuba@kernel.org>