selftests: mptcp: userspace_pm: unmute unexpected errors

All pm_nl_ctl commands were muted. If there was an unexpected error with
one of them, this was simply not visible in the logs, making the
analysis very hard. It could also hide misuse of commands by mistake.

Now the output is only muted when we do expect to have an error, e.g.
when giving invalid arguments on purpose.
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20230730-upstream-net-next-20230728-mptcp-selftests-misc-v1-4-7e9cc530a9cd@tessares.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>