selftests: pmtu: Simplify cleanup and namespace names

The point of the pause-on-fail argument is to leave the setup as is after
a test fails to allow a user to debug why it failed. Move the cleanup
after posting the result to the user to make it so.

Random names for the namespaces are not user friendly when trying to
debug a failure. Make them simpler and more direct for the tests. Run
cleanup at the beginning to ensure they are cleaned up if they already
exist.

Remove cleanup_done. There is no harm in doing cleanup twice; just
ignore any errors related to not existing - which is already done.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>