rtc: sun6i: Use struct_size() in kzalloc()

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct foo {
	int stuff;
        void *entry[];
};

instance = kzalloc(sizeof(struct foo) + sizeof(void *) * count, GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL);
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-sun6i.c

@@ -199,8 +199,7 @@ static void __init sun6i_rtc_clk_init(struct device_node *node)
 	if (!rtc)
 		return;
 
-	clk_data = kzalloc(sizeof(*clk_data) + (sizeof(*clk_data->hws) * 2),
-			   GFP_KERNEL);
+	clk_data = kzalloc(struct_size(clk_data, hws, 2), GFP_KERNEL);
 	if (!clk_data) {
 		kfree(rtc);
 		return;