iommu/qcom: Use struct_size() helper

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 qcom_iommu_dev {
	...
        struct qcom_iommu_ctx   *ctxs[0];   /* indexed by asid-1 */
};

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

So, replace the following form:

sizeof(*qcom_iommu) + (max_asid * sizeof(qcom_iommu->ctxs[0]))

with:

struct_size(qcom_iommu, ctxs, max_asid)

Also, notice that, in this case, variable 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: Joerg Roedel <jroedel@suse.de>

drivers/iommu/qcom_iommu.c

@@ -775,7 +775,7 @@ static int qcom_iommu_device_probe(struct platform_device *pdev)
 	struct qcom_iommu_dev *qcom_iommu;
 	struct device *dev = &pdev->dev;
 	struct resource *res;
-	int ret, sz, max_asid = 0;
+	int ret, max_asid = 0;
 
 	/* find the max asid (which is 1:1 to ctx bank idx), so we know how
 	 * many child ctx devices we have:
@@ -783,9 +783,8 @@ static int qcom_iommu_device_probe(struct platform_device *pdev)
 	for_each_child_of_node(dev->of_node, child)
 		max_asid = max(max_asid, get_asid(child));
 
-	sz = sizeof(*qcom_iommu) + (max_asid * sizeof(qcom_iommu->ctxs[0]));
-
-	qcom_iommu = devm_kzalloc(dev, sz, GFP_KERNEL);
+	qcom_iommu = devm_kzalloc(dev, struct_size(qcom_iommu, ctxs, max_asid),
+				  GFP_KERNEL);
 	if (!qcom_iommu)
 		return -ENOMEM;
 	qcom_iommu->num_ctxs = max_asid;