memory-barriers: Fix control-ordering no-transitivity example

The control-ordering example demonstrating lack of transitivity had
multiple problems.  This commit fixes them.

Reported-by: Nikolay Samofatov <nikolay.samofatov@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>