rcu: Mark sync_sched_exp_online_cleanup() ->cpu_no_qs.b.exp load

The sync_sched_exp_online_cleanup() is called from rcutree_online_cpu(),
which can be invoked with interrupts enabled.  This means that
the ->cpu_no_qs.b.exp field is subject to data races from the
rcu_exp_handler() IPI handler, so this commit marks the load from
that field.

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

kernel/rcu/tree_exp.h

@@ -759,7 +759,7 @@ static void sync_sched_exp_online_cleanup(int cpu)
 	my_cpu = get_cpu();
 	/* Quiescent state either not needed or already requested, leave. */
 	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
-	    rdp->cpu_no_qs.b.exp) {
+	    READ_ONCE(rdp->cpu_no_qs.b.exp)) {
 		put_cpu();
 		return;
 	}