Revert "drm/virtio: Use dma_buf from GEM object instance"

This reverts commit 415cb45.

The dma_buf field in struct drm_gem_object is not stable over the
object instance's lifetime. The field becomes NULL when user space
releases the final GEM handle on the buffer object. This resulted
in a NULL-pointer deref.

Workarounds in commit 5307dce ("drm/gem: Acquire references on
GEM handles for framebuffers") and commit f6bfc9a ("drm/framebuffer:
Acquire internal references on GEM handles") only solved the problem
partially. They especially don't work for buffer objects without a DRM
framebuffer associated.

Hence, this revert to going back to using .import_attach->dmabuf.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://lore.kernel.org/r/20250715155934.150656-3-tzimmermann@suse.de

Author: tdz

drivers/gpu/drm/virtio/virtgpu_prime.c

@@ -204,15 +204,16 @@ static void virtgpu_dma_buf_free_obj(struct drm_gem_object *obj)
 {
 	struct virtio_gpu_object *bo = gem_to_virtio_gpu_obj(obj);
 	struct virtio_gpu_device *vgdev = obj->dev->dev_private;
+	struct dma_buf_attachment *attach = obj->import_attach;
 
 	if (drm_gem_is_imported(obj)) {
-		struct dma_buf *dmabuf = obj->dma_buf;
+		struct dma_buf *dmabuf = attach->dmabuf;
 
 		dma_resv_lock(dmabuf->resv, NULL);
 		virtgpu_dma_buf_unmap(bo);
 		dma_resv_unlock(dmabuf->resv);
 
-		dma_buf_detach(dmabuf, obj->import_attach);
+		dma_buf_detach(dmabuf, attach);
 		dma_buf_put(dmabuf);
 	}