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

One of them is the support of TCP_FASTOPEN socket option with MPTCP
connections introduced by commit 4ffb0a02 ("mptcp: add TCP_FASTOPEN
sock option").

It is possible to look for "mptcp_fastopen_" in kallsyms to know if the
feature is supported or not.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: ca7ae891 ("selftests: mptcp: mptfo Initiator/Listener")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>