Merge branch 'mptcp-make-add_addr-retransmission-timeout-adaptive'

Matthieu Baerts says:

====================
mptcp: make ADD_ADDR retransmission timeout adaptive

Currently, the MPTCP ADD_ADDR notifications are retransmitted after a
fixed timeout controlled by the net.mptcp.add_addr_timeout sysctl knob,
if the corresponding "echo" packets are not received before. This can be
too slow (or too quick), especially with a too cautious default value
set to 2 minutes.

- Patch 1: make ADD_ADDR retransmission timeout adaptive, using the
  TCP's retransmission timeout. The corresponding sysctl knob is now
  used as a maximum value.

- Patch 2: now that these ADD_ADDR retransmissions can happen faster,
  all MPTCP Join subtests checking ADD_ADDR counters accept more
  ADD_ADDR than expected (if any). This is aligned with the previous
  behaviour, when the ADD_ADDR RTO was lowered down to 1 second.

- Patch 3: Some CIs have reported that some MPTCP Join signalling tests
  were unstable. It seems that it is due to the time it can take in slow
  environments to send a bunch of ADD_ADDR notifications and wait each
  time for their echo reply. Use a longer transfer to avoid such errors.

v1: https://lore.kernel.org/d5397026-92eb-4a43-9534-954b43ab9305@kernel.org
====================

Link: https://patch.msgid.link/20250907-net-next-mptcp-add_addr-retrans-adapt-v1-0-824cc805772b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>