selftests: mptcp: lib: skip if missing symbol

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

New functions are now available to easily detect if a certain feature is
missing by looking at kallsyms.

These new helpers are 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>