drm/nouveau/mmu: 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(*kind) + sizeof(*kind->data) * mmu->kind_nr;

with:

struct_size(kind, data, mmu->kind_nr)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvif/mmu.c

@@ -110,7 +110,7 @@ nvif_mmu_init(struct nvif_object *parent, s32 oclass, struct nvif_mmu *mmu)
 
 	if (mmu->kind_nr) {
 		struct nvif_mmu_kind_v0 *kind;
-		u32 argc = sizeof(*kind) + sizeof(*kind->data) * mmu->kind_nr;
+		size_t argc = struct_size(kind, data, mmu->kind_nr);
 
 		if (ret = -ENOMEM, !(kind = kmalloc(argc, GFP_KERNEL)))
 			goto done;