rcu: Make use of rcu_preempt_has_tasks()

Given that there is now arcu_preempt_has_tasks() function that checks
to see if the ->blkd_tasks list is non-empty, this commit makes use of it.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree_plugin.h

@@ -540,7 +540,7 @@ static int rcu_print_task_stall(struct rcu_node *rnp)
 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 {
 	WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
-	if (!list_empty(&rnp->blkd_tasks))
+	if (rcu_preempt_has_tasks(rnp))
 		rnp->gp_tasks = rnp->blkd_tasks.next;
 	WARN_ON_ONCE(rnp->qsmask);
 }
@@ -706,7 +706,7 @@ sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
 
 	raw_spin_lock_irqsave(&rnp->lock, flags);
 	smp_mb__after_unlock_lock();
-	if (list_empty(&rnp->blkd_tasks)) {
+	if (!rcu_preempt_has_tasks(rnp)) {
 		raw_spin_unlock_irqrestore(&rnp->lock, flags);
 	} else {
 		rnp->exp_tasks = rnp->blkd_tasks.next;
@@ -985,7 +985,7 @@ void exit_rcu(void)
 
 static void rcu_initiate_boost_trace(struct rcu_node *rnp)
 {
-	if (list_empty(&rnp->blkd_tasks))
+	if (!rcu_preempt_has_tasks(rnp))
 		rnp->n_balk_blkd_tasks++;
 	else if (rnp->exp_tasks == NULL && rnp->gp_tasks == NULL)
 		rnp->n_balk_exp_gp_tasks++;