selftests: mptcp: lib: skip if missing symbol

stable inclusion
from stable-v5.10.186
commit 6d20cfbc578d01d87cc1b1610d32ce1d1d0d8480
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8L5XP

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d20cfbc578d01d87cc1b1610d32ce1d1d0d8480

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

commit 67300482 upstream.

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

New functions are now available to easily detect if a certain feature is
missing by looking at kallsyms.

These new helpers are going to be used in the following commits. In
order to ease the backport of such future patches, it would be good if
this patch is backported up to the introduction of MPTCP selftests,
hence the Fixes tag below: this type of check was supposed to be done
from the beginning.

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


Fixes: 048d19d4 ("mptcp: add basic kselftest for mptcp")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>