selftests: mptcp: join: skip mixed 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 a mix of subflows in v4 and v6 by the
in-kernel PM introduced by commit b9d69db8 ("mptcp: let the
in-kernel PM use mixed IPv4 and IPv6 addresses").

It looks like there is no external sign we can use to predict the
expected behaviour. Instead of accepting different behaviours and thus
not really checking for the expected behaviour, we are looking here for
a specific kernel version. That's not ideal but it looks better than
removing the test because it cannot support older kernel versions.

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


Fixes: ad349374 ("selftests: mptcp: add test-cases for mixed v4/v6 subflows")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>