serial: max310x: 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 code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/max310x.c

@@ -1197,8 +1197,7 @@ static int max310x_probe(struct device *dev, struct max310x_devtype *devtype,
 		return PTR_ERR(regmap);
 
 	/* Alloc port structure */
-	s = devm_kzalloc(dev, sizeof(*s) +
-			 sizeof(struct max310x_one) * devtype->nr, GFP_KERNEL);
+	s = devm_kzalloc(dev, struct_size(s, p, devtype->nr), GFP_KERNEL);
 	if (!s) {
 		dev_err(dev, "Error allocating port structure\n");
 		return -ENOMEM;