Merge branch 'mptcp-mib-counters-for-mpj-tx-misc-improvements'

Matthieu Baerts says:

====================
mptcp: MIB counters for MPJ TX + misc improvements

Recently, a few issues have been discovered around the creation of
additional subflows. Without these counters, it was difficult to point
out the reason why some subflows were not created as expected.

In patch 3, all error paths from __mptcp_subflow_connect() are covered,
except the one related to the 'fully established mode', because it can
only happen with the userspace PM, which will propagate the error to the
userspace in this case (ENOTCONN).

These new counters are also verified in the MPTCP Join selftest in patch
6.

While at it, a few other patches are improving the MPTCP path-manager
code ...

 - Patch 1: 'flush' related helpers are renamed to avoid confusions
 - Patch 2: directly pass known ID and flags to create a new subflow,
            i/o getting them later by iterating over all endpoints again

... and the MPJoin selftests:

 - Patch 4: reduce the number of positional parameters
 - Patch 5: only one line for the 'join' checks, instead of 3
 - Patch 7: more explicit check names, instead of sometimes too cryptic
            ones: rtx, ptx, ftx, ctx, fclzrx, sum
 - Patch 8: specify client/server instead of 'invert' for some checks
            not suggesting one specific direction
 - Patch 9: mute errors of mptcp_connect when ran in the background
 - Patch 10: simplify checksum_tests by using a for-loop
 - Patch 11: remove 'define' re-definitions
====================

Link: https://patch.msgid.link/20240902-net-next-mptcp-mib-mpjtx-misc-v1-0-d3e0f3773b90@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>