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

One of them is the MPTCP Userspace PM introduced by commit 4638de5a
("mptcp: handle local addrs announced by userspace PMs").

We can skip all these tests if the feature is not supported simply by
looking for the MPTCP pm_type's sysctl knob.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 259a834f ("selftests: mptcp: functional tests for the 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>