selftests: mptcp: join: skip check if MIB counter not supported

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

One of them is the MPTCP MIB counters introduced in commit fc51895
("mptcp: add and use MIB counter infrastructure") and more later. The
MPTCP Join selftest heavily relies on these counters.

If a counter is not supported by the kernel, it is not displayed when
using 'nstat -z'. We can then detect that and skip the verification. A
new helper (get_counter()) has been added to do the required checks and
return an error if the counter is not available.

Note that if we expect to have these features available and if
SELFTESTS_MPTCP_LIB_EXPECT_ALL_FEATURES env var is set to 1, the tests
will be marked as failed instead of skipped.

This new helper also makes sure we get the exact counter we want to
avoid issues we had in the past, e.g. with MPTcpExtRmAddr and
MPTcpExtRmAddrDrop sharing the same prefix. While at it, we uniform the
way we fetch a MIB counter.

Note for the backports: we rarely change these modified blocks so if
there is are conflicts, it is very likely because a counter is not used
in the older kernels and we don't need that chunk.

Link: #368
Fixes: b08fbf2 ("selftests: add test-cases for MPTCP MP_JOIN")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>