drm/sun4i: tcon-top: Use struct_size() in devm_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 = devm_kzalloc(dev, 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 = devm_kzalloc(dev, struct_size(instance, entry, count), GFP_KERNEL);

This issue was detected with the help of Coccinelle.
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180824010521.GA25451@embeddedor.com

drivers/gpu/drm/sun4i/sun8i_tcon_top.c

@@ -129,8 +129,7 @@ static int sun8i_tcon_top_bind(struct device *dev, struct device *master,
 	if (!tcon_top)
 		return -ENOMEM;
 
-	clk_data = devm_kzalloc(dev, sizeof(*clk_data) +
-				sizeof(*clk_data->hws) * CLK_NUM,
+	clk_data = devm_kzalloc(dev, struct_size(clk_data, hws, CLK_NUM),
 				GFP_KERNEL);
 	if (!clk_data)
 		return -ENOMEM;