selftests: mptcp: join: restrict fullmesh endp on 1st sf

commit 49ac6f05 upstream.

A new endpoint using the IP of the initial subflow has been recently
added to increase the code coverage. But it breaks the test when using
old kernels not having commit 86e39e04 ("mptcp: keep track of local
endpoint still available for each msk"), e.g. on v5.15.

Similar to commit d4c81bbb ("selftests: mptcp: join: support local
endpoint being tracked or not"), it is possible to add the new endpoint
conditionally, by checking if "mptcp_pm_subflow_check_next" is present
in kallsyms: this is not directly linked to the commit introducing this
symbol but for the parent one which is linked anyway. So we can know in
advance what will be the expected behaviour, and add the new endpoint
only when it makes sense to do so.

Fixes: 4878f9f8 ("selftests: mptcp: join: validate fullmesh endp on 1st sf")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240910-net-selftests-mptcp-fix-install-v1-1-8f124aa9156d@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Conflicts in mptcp_join.sh, because the 'run_tests' helper has been
  modified in multiple commits that are not in this version, e.g. commit
  e571fb09 ("selftests: mptcp: add speed env var"). The conflict was
  in the context, the new lines can still be added at the same place. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>