Merge branch 'mptcp-autotune-related-improvement'

Matthieu Baerts says:

====================
mptcp: autotune related improvement

Here are two patches from Paolo that have been crafted a couple of
months ago, but needed more validation because they were indirectly
causing instabilities in the sefltests. The root cause has been fixed in
'net' recently in commit 8c09412e584d ("selftests: mptcp: more stable
simult_flows tests").

These patches refactor the receive space and RTT estimator, overall
making DRS more correct while avoiding receive buffer drifting to
tcp_rmem[2], which in turn makes the throughput more stable and less
bursty, especially with high bandwidth and low delay environments.

Note that the first patch addresses a very old issue. 'net-next' is
targeted because the change is quite invasive and based on a recent
backlog refactor. The 'Fixes' tag is then there more as a FYI, because
backporting this patch will quickly be blocked due to large conflicts.
====================

Link: https://patch.msgid.link/20260407-net-next-mptcp-reduce-rbuf-v2-0-0d1d135bf6f6@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>