xen: use struct_size() helper in kzalloc()

struct privcmd_buf_vma_private has a zero-sized array at the end
(pages), use the new struct_size() helper to determine the proper
allocation size and avoid potential type mistakes.

Signed-off-by: Andrea Righi <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Signed-off-by: Juergen Gross <[e-mail]>

drivers/xen/privcmd-buf.c

···
 	if (!(vma->vm_flags & VM_SHARED))
 		return -EINVAL;
 
-	vma_priv = kzalloc(sizeof(*vma_priv) + count * sizeof(void *),
-			   GFP_KERNEL);
+	vma_priv = kzalloc(struct_size(vma_priv, pages, count), GFP_KERNEL);
 	if (!vma_priv)
 		return -ENOMEM;