selftests: mptcp: join: skip userspace PM tests if not supported

selftests: mptcp: join: skip userspace PM tests if not supported

BugLink: https://bugs.launchpad.net/bugs/2033931
commit f2b492b04a167261e1c38eb76f78fb4294473a49 upstream.

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 userspace PM introduced by commit
4638de5aefe5 ("mptcp: handle local addrs announced by userspace PMs")
and the following ones.

It is possible to look for the MPTCP pm_type's sysctl knob to know in
advance if the userspace PM is available.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 5ac1d2d63451 ("selftests: mptcp: Add tests for userspace PM type")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>