selftests: mptcp: join: support RM_ADDR for used endpoints or not

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

At some points, a new feature caused internal behaviour changes we are
verifying in the selftests, see the Fixes tag below. It was not a UAPI
change but because in these selftests, we check some internal
behaviours, it is normal we have to adapt them from time to time after
having added some features.

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: 6fa0174a

 ("mptcp: more careful RM_ADDR generation")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>