selftests: mptcp: lib: skip if not below kernel version

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

A new function is now available to easily detect if a feature is
missing by looking at the kernel version. That's clearly not ideal and
this kind of check should be avoided as soon as possible. But sometimes,
there are no external sign that a "feature" is available or not:
internal behaviours can change without modifying the uAPI and these
selftests are verifying the internal behaviours. Sometimes, the only
(easy) way to verify if the feature is present is to run the test but
then the validation cannot determine if there is a failure with the
feature or if the feature is missing. Then it looks better to check the
kernel version instead of having tests that can never fail. In any case,
we need a solution not to have a whole selftest being marked as failed
just because one sub-test has failed.

Note that this env var car be set to 1 not to do such check and run the
linked sub-test: SELFTESTS_MPTCP_LIB_NO_KVERSION_CHECK.

This new helper is going to be used in the following commits. In order
to ease the backport of such future patches, it would be good if this
patch is backported up to the introduction of MPTCP selftests, hence the
Fixes tag below: this type of check was supposed to be done from the
beginning.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 048d19d4

 ("mptcp: add basic kselftest for mptcp")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>