rcuperf: Fix printk format warning

Using "%zu" to fix following warning,
kernel/rcu/rcuperf.c: In function ‘kfree_perf_init’:
include/linux/kern_levels.h:5:18: warning: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 2 has type ‘unsigned int’ [-Wformat=]
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcuperf.c

@@ -723,7 +723,7 @@ kfree_perf_init(void)
 		schedule_timeout_uninterruptible(1);
 	}
 
-	pr_alert("kfree object size=%lu\n", kfree_mult * sizeof(struct kfree_obj));
+	pr_alert("kfree object size=%zu\n", kfree_mult * sizeof(struct kfree_obj));
 
 	kfree_reader_tasks = kcalloc(kfree_nrealthreads, sizeof(kfree_reader_tasks[0]),
 			       GFP_KERNEL);