selftests: mptcp: userspace_pm: report errors with 'remove' tests

A message was mentioning an issue with the "remove" tests but the
selftest was not marked as failed.

Directly exit with an error like it is done everywhere else in this
selftest.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 259a834f

 ("selftests: mptcp: functional tests for the userspace PM type")
Cc: stable@vger.kernel.org
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>