selftests: mptcp: sockopt: skip getsockopt 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 the getsockopt(SOL_MPTCP) to get info about the MPTCP
connections introduced by commit 55c42fa7 ("mptcp: add MPTCP_INFO
getsockopt") and the following ones.

It is possible to look for "mptcp_diag_fill_info" in kallsyms because
it is introduced by the mentioned feature. So we can know in advance if
the feature is supported and skip the sub-test if not.

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


Fixes: ce997912 ("selftests: mptcp: add mptcp getsockopt test cases")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>