rcu: Make preemptible rcu_exp_handler() check idempotency

Although the non-preemptible implementation of rcu_exp_handler()
contains checks to enforce idempotency, the preemptible version does not.
The reason for this omission is that in preemptible kernels, there is
no reporting of quiescent states from CPU hotplug notifiers, and thus
no need for idempotency.

In theory, anyway.

In practice, accidents happen.  This commit therefore adds checks under
WARN_ON_ONCE() to catch any such accidents.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>