Input: gpio_keys - 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 gpio_keys_drvdata {
	...
        struct gpio_button_data data[0];
};

size = sizeof(struct gpio_keys_drvdata) + count * sizeof(struct gpio_button_data);
instance = devm_kzalloc(dev, size, 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, data, count), GFP_KERNEL);

Notice that, in this case, variable size is not necessary, hence it
is removed.

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/keyboard/gpio_keys.c

@@ -774,7 +774,6 @@ static int gpio_keys_probe(struct platform_device *pdev)
 	struct fwnode_handle *child = NULL;
 	struct gpio_keys_drvdata *ddata;
 	struct input_dev *input;
-	size_t size;
 	int i, error;
 	int wakeup = 0;
 
@@ -784,9 +783,8 @@ static int gpio_keys_probe(struct platform_device *pdev)
 			return PTR_ERR(pdata);
 	}
 
-	size = sizeof(struct gpio_keys_drvdata) +
-			pdata->nbuttons * sizeof(struct gpio_button_data);
-	ddata = devm_kzalloc(dev, size, GFP_KERNEL);
+	ddata = devm_kzalloc(dev, struct_size(ddata, data, pdata->nbuttons),
+			     GFP_KERNEL);
 	if (!ddata) {
 		dev_err(dev, "failed to allocate state\n");
 		return -ENOMEM;