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 MP_FAIL / infinite mapping introduced
by commit 1e39e5a3 ("mptcp: infinite mapping sending") and the
following ones.

It is possible to look for one of the infinite mapping counters to know
in advance if the this feature is available.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: b6e074e1 ("selftests: mptcp: add infinite map testcase")
Cc: stable@vger.kernel.org
Fixes: 2ba18161 ("selftests: mptcp: add MP_FAIL reset testcase")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>