rcu: Mark ->expmask access in synchronize_rcu_expedited_wait()

This commit adds a READ_ONCE() to an access to the rcu_node structure's
->expmask field to prevent compiler mischief.  Detected by KCSAN.

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

Author: paulmckrcu

kernel/rcu/tree_exp.h

@@ -502,7 +502,8 @@ static void synchronize_rcu_expedited_wait(void)
 		if (synchronize_rcu_expedited_wait_once(1))
 			return;
 		rcu_for_each_leaf_node(rnp) {
-			for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
+			mask = READ_ONCE(rnp->expmask);
+			for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
 				rdp = per_cpu_ptr(&rcu_data, cpu);
 				if (rdp->rcu_forced_tick_exp)
 					continue;