irqchip/qcom: 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);

Notice that, in this case, variable alloc_sz 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: Marc Zyngier <marc.zyngier@arm.com>

drivers/irqchip/qcom-irq-combiner.c

@@ -237,7 +237,6 @@ static int get_registers(struct platform_device *pdev, struct combiner *comb)
 static int __init combiner_probe(struct platform_device *pdev)
 {
 	struct combiner *combiner;
-	size_t alloc_sz;
 	int nregs;
 	int err;
 
@@ -247,8 +246,8 @@ static int __init combiner_probe(struct platform_device *pdev)
 		return -EINVAL;
 	}
 
-	alloc_sz = sizeof(*combiner) + sizeof(struct combiner_reg) * nregs;
-	combiner = devm_kzalloc(&pdev->dev, alloc_sz, GFP_KERNEL);
+	combiner = devm_kzalloc(&pdev->dev, struct_size(combiner, regs, nregs),
+				GFP_KERNEL);
 	if (!combiner)
 		return -ENOMEM;