selftests: mptcp: join: skip fullmesh flag tests if not supported

Selftests are supposed to run on any kernels, including the old ones not
supporting all MPTCP features.

One of them is the support of the fullmesh flag for the in-kernel PM
introduced by commit 2843ff6f ("mptcp: remote addresses fullmesh")
and commit 1a0d6136 ("mptcp: local addresses fullmesh").

It looks like there is no easy external sign we can use to predict the
expected behaviour. We could add the flag and then check if it has been
added but for that, and for each fullmesh test, we would need to setup a
new environment, do the checks, clean it and then only start the test
from yet another clean environment. To keep it simple and avoid
introducing new issues, we look for a specific kernel version. That's
not ideal but an acceptable solution for this case.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 6a0653b9

 ("selftests: mptcp: add fullmesh setting tests")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>