selftests: mptcp: join: skip check if MIB counter not supported (part 2)

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 fc518953
("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 recently in the -net tree to
do the required checks and return an error if the counter is not
available.

This commit is similar to the one with the same title applied in the
-net tree but it modifies code only present in net-next for the moment,
see the Fixes commit below.

While at it, we can also remove the use of ${extra_msg} variable which
is never assigned in chk_rm_tx_nr() function and use 'echo' without '-n'
parameter.

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


Fixes: 0639fa23 ("selftests: mptcp: add explicit check for new mibs")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>