rcu: Ignore rdp.cpu_no_qs.b.exp on preemptible RCU's rcu_qs()

Preemptible RCU does not use the rcu_data structure's ->cpu_no_qs.b.exp,
instead using a separate ->exp_deferred_qs field to record the need for
an expedited quiescent state.

In fact ->cpu_no_qs.b.exp should never be set in preemptible RCU because
preemptible RCU's expedited grace periods use other mechanisms to record
quiescent states.

This commit therefore removes the implicit rcu_qs() reference to
->cpu_no_qs.b.exp in favor of a direct reference to ->cpu_no_qs.b.norm.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>