Input: tca6416-keypad - 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);

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/tca6416-keypad.c

@@ -219,9 +219,7 @@ static int tca6416_keypad_probe(struct i2c_client *client,
 		return -EINVAL;
 	}
 
-	chip = kzalloc(sizeof(struct tca6416_keypad_chip) +
-		       pdata->nbuttons * sizeof(struct tca6416_button),
-		       GFP_KERNEL);
+	chip = kzalloc(struct_size(chip, buttons, pdata->nbuttons), GFP_KERNEL);
 	input = input_allocate_device();
 	if (!chip || !input) {
 		error = -ENOMEM;