genirq/devres: 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[];
};

size = sizeof(struct foo) + count * sizeof(struct boo);
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, 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: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190410170914.GA16161@embeddedor

kernel/irq/devres.c

@@ -220,9 +220,8 @@ devm_irq_alloc_generic_chip(struct device *dev, const char *name, int num_ct,
 			    irq_flow_handler_t handler)
 {
 	struct irq_chip_generic *gc;
-	unsigned long sz = sizeof(*gc) + num_ct * sizeof(struct irq_chip_type);
 
-	gc = devm_kzalloc(dev, sz, GFP_KERNEL);
+	gc = devm_kzalloc(dev, struct_size(gc, chip_types, num_ct), GFP_KERNEL);
 	if (gc)
 		irq_init_generic_chip(gc, name, num_ct,
 				      irq_base, reg_base, handler);