tests/rcutorture: mild documenting refactor of update thread

This is mainly to help with reasoning what the test is trying to do.
We can move rcu_stress_idx to a local variable as there is only ever
one updater thread. I've also added an assert to catch the case where
we end up updating the current structure to itself which is the only
way I can see the mberror cases we are seeing on Travis.

We shall see if the rcutorture test failures go away now.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Message-Id: <20200225124710.14152-5-alex.bennee@linaro.org>

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