rcu: Remove redundant ACCESS_ONCE() from tick_do_timer_cpu

In kernels built with CONFIG_NO_HZ_FULL, tick_do_timer_cpu is constant
once boot completes.  Thus, there is no need to wrap it in ACCESS_ONCE()
in code that is built only when CONFIG_NO_HZ_FULL.  This commit therefore
removes the redundant ACCESS_ONCE().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>

kernel/rcu/tree_plugin.h

@@ -2844,7 +2844,7 @@ static bool rcu_nohz_full_cpu(struct rcu_state *rsp)
 static void rcu_bind_gp_kthread(void)
 {
 #ifdef CONFIG_NO_HZ_FULL
-	int cpu = ACCESS_ONCE(tick_do_timer_cpu);
+	int cpu = tick_do_timer_cpu;
 
 	if (cpu < 0 || cpu >= nr_cpu_ids)
 		return;