drm/i915/kvmgt: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, replace the following form:

sizeof(*sparse) + (nr_areas * sizeof(*sparse->areas)

with:

struct_size(sparse, areas, sparse->nr_areas)

and so on...

Also, notice that variable size is unnecessary, 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: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/kvmgt.c

@@ -1306,7 +1306,6 @@ static long intel_vgpu_ioctl(struct mdev_device *mdev, unsigned int cmd,
 		unsigned int i;
 		int ret;
 		struct vfio_region_info_cap_sparse_mmap *sparse = NULL;
-		size_t size;
 		int nr_areas = 1;
 		int cap_type_id;
 
@@ -1349,9 +1348,8 @@ static long intel_vgpu_ioctl(struct mdev_device *mdev, unsigned int cmd,
 					VFIO_REGION_INFO_FLAG_WRITE;
 			info.size = gvt_aperture_sz(vgpu->gvt);
 
-			size = sizeof(*sparse) +
-					(nr_areas * sizeof(*sparse->areas));
-			sparse = kzalloc(size, GFP_KERNEL);
+			sparse = kzalloc(struct_size(sparse, areas, nr_areas),
+					 GFP_KERNEL);
 			if (!sparse)
 				return -ENOMEM;
 
@@ -1416,9 +1414,9 @@ static long intel_vgpu_ioctl(struct mdev_device *mdev, unsigned int cmd,
 			switch (cap_type_id) {
 			case VFIO_REGION_INFO_CAP_SPARSE_MMAP:
 				ret = vfio_info_add_capability(&caps,
-					&sparse->header, sizeof(*sparse) +
-					(sparse->nr_areas *
-						sizeof(*sparse->areas)));
+					&sparse->header,
+					struct_size(sparse, areas,
+						    sparse->nr_areas));
 				if (ret) {
 					kfree(sparse);
 					return ret;