mfd: sm501: 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;
    struct boo entry[];
};

instance = devm_kzalloc(dev, sizeof(struct foo) + count * sizeof(struct boo), 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 code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/sm501.c

@@ -1142,8 +1142,7 @@ static int sm501_register_gpio_i2c_instance(struct sm501_devdata *sm,
 		return -ENOMEM;
 
 	/* Create a gpiod lookup using gpiochip-local offsets */
-	lookup = devm_kzalloc(&pdev->dev,
-			      sizeof(*lookup) + 3 * sizeof(struct gpiod_lookup),
+	lookup = devm_kzalloc(&pdev->dev, struct_size(lookup, table, 3),
 			      GFP_KERNEL);
 	if (!lookup)
 		return -ENOMEM;