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

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

commit 966c6c3adfb1257ea8a839cdfad2b74092cc5532 upstream.

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: 259a834fadda ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>