drm/virtio: fix OOB in virtio_gpu_object_create

After commit f651c8b05542 ("drm/virtio: factor out the sg_table from
virtio_gpu_object"), virtio_gpu_create_object allocates too small space
to fit everything in. It is because it allocates struct
virtio_gpu_object, but should allocate a newly added struct
virtio_gpu_object_shmem which has 2 more members.

So fix that by using correct type in virtio_gpu_create_object.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: http://patchwork.freedesktop.org/patch/msgid/20200319100421.16267-1-jslaby@suse.cz
Fixes: f651c8b05542 ("drm/virtio: factor out the sg_table from virtio_gpu_object")
Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 0666a8d7f6a4530440e59f2d22ed4091f4d3818c)
diff --git a/drivers/gpu/drm/virtio/virtgpu_object.c b/drivers/gpu/drm/virtio/virtgpu_object.c
index 2bfb13d..d9039bb 100644
--- a/drivers/gpu/drm/virtio/virtgpu_object.c
+++ b/drivers/gpu/drm/virtio/virtgpu_object.c
@@ -123,15 +123,17 @@
 struct drm_gem_object *virtio_gpu_create_object(struct drm_device *dev,
 						size_t size)
 {
-	struct virtio_gpu_object *bo;
+	struct virtio_gpu_object_shmem *shmem;
+	struct drm_gem_shmem_object *dshmem;
 
-	bo = kzalloc(sizeof(*bo), GFP_KERNEL);
-	if (!bo)
+	shmem = kzalloc(sizeof(*shmem), GFP_KERNEL);
+	if (!shmem)
 		return NULL;
 
-	bo->base.base.funcs = &virtio_gpu_shmem_funcs;
-	bo->base.map_cached = true;
-	return &bo->base.base;
+	dshmem = &shmem->base.base;
+	dshmem->base.funcs = &virtio_gpu_shmem_funcs;
+	dshmem->map_cached = true;
+	return &dshmem->base;
 }
 
 static int virtio_gpu_object_shmem_init(struct virtio_gpu_device *vgdev,