selftests: mptcp: join: skip backup if set flag on ID not supported

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

Commit bccefb76 ("selftests: mptcp: simplify pm_nl_change_endpoint")
has simplified the way the backup flag is set on an endpoint. Instead of
doing:

  ./pm_nl_ctl set 10.0.2.1 flags backup

Now we do:

  ./pm_nl_ctl set id 1 flags backup

The new way is easier to maintain but it is also incompatible with older
kernels not supporting the implicit endpoints putting in place the
infrastructure to set flags per ID, hence the second Fixes tag.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: bccefb76 ("selftests: mptcp: simplify pm_nl_change_endpoint")
Cc: stable@vger.kernel.org
Fixes: 4cf86ae8

 ("mptcp: strict local address ID selection")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>