selftests: mptcp: join: skip if MPTCP is not supported

stable inclusion
from stable-v5.10.183
commit 92450a1eaa9e570d296bf86e9719a95ebe853f18
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8IRR2

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=92450a1eaa9e570d296bf86e9719a95ebe853f18

--------------------------------

commit 715c78a8 upstream.

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

A new check is then added to make sure MPTCP is supported. If not, the
test stops and is marked as "skipped".

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


Fixes: b08fbf24 ("selftests: add test-cases for MPTCP MP_JOIN")
Cc: stable@vger.kernel.org
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>