selftests: mptcp: sockopt: skip TCP_INQ checks if not supported

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

One of them is TCP_INQ cmsg support introduced in commit 2c9e7765
("mptcp: add TCP_INQ cmsg support").

It is possible to look for "mptcp_ioctl" in kallsyms because it was
needed to introduce the mentioned feature. We can skip these tests and
not set TCPINQ option if the feature is not supported.

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


Fixes: 5cbd886c ("selftests: mptcp: add TCP_INQ support")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>