rcuperf: Further fix printk format warning

This commit casts the value to size_t in order to allow %zu to
operate across all architectures.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
index 962869d..dc7483a 100644
--- a/kernel/rcu/rcuperf.c
+++ b/kernel/rcu/rcuperf.c
@@ -724,7 +724,7 @@ kfree_perf_init(void)
 		schedule_timeout_uninterruptible(1);
 	}
 
-	pr_alert("kfree object size=%zu\n", kfree_mult * sizeof(struct kfree_obj));
+	pr_alert("kfree object size=%zu\n", (size_t)kfree_mult * sizeof(struct kfree_obj));
 
 	kfree_reader_tasks = kcalloc(kfree_nrealthreads, sizeof(kfree_reader_tasks[0]),
 			       GFP_KERNEL);