tools/memory-model: Use "buf" and "flag" for message-passing tests

tools/memory-model: Use "buf" and "flag" for message-passing tests

The use of "x" and "y" for message-passing tests is fine for people
familiar with memory models and litmus-test nomenclature, but is a bit
obtuse for others.  This commit therefore substitutes "buf" for "x" and
"flag" for "y" for the MP tests.  There are a few special-case MP tests
that use locks and these are unchanged.  There is another MP test that
uses pointers, and this is changed to name the pointer "p".

Reported-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>