selftests: mptcp: connect: skip transp tests if not supported

selftests: mptcp: connect: skip transp tests 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 support of IP(V6)_TRANSPARENT socket option with
MPTCP connections introduced by commit c9406a23c116 ("mptcp: sockopt:
add SOL_IP freebind & transparent options").

It is possible to look for "__ip_sock_set_tos" in kallsyms because
IP(V6)_TRANSPARENT socket option support has been added after TOS
support which came with the required infrastructure in MPTCP sockopt
code. To support TOS, the following function has been exported (T). Not
great but better than checking for a specific kernel version.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 5fb62e9cd3ad ("selftests: mptcp: add tproxy test case")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>