selftests: mptcp: userspace_pm: fix shellcheck warnings

shellcheck recently helped to find an issue where a wrong variable name
was used. It is then good to fix the other harmless issues in order to
spot "real" ones later.

Here, three categories of warnings are ignored:

- SC2317: Command appears to be unreachable. The cleanup() function is
  invoke indirectly via the EXIT trap.

- SC2034: Variable appears unused. The check_expected_one() function
  takes the name of the variable in argument but it ends up reading the
  content: indirect usage.

- SC2086: Double quote to prevent globbing and word splitting. This is
  recommended but the current usage is correct and there is no need to
  do all these modifications to be compliant with this rule.

One error has been fixed with SC2181: Check exit code directly with e.g.
'if ! mycmd;', not indirectly with $?.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>