drm/vgem: Ditch attach trickery in the fence ioctl

It looks like this was done purely to get a consistent place to look
up the reservation object pointer. With the drm_prime.c helper code
now also setting gem_object->resv for imported objects we can just use
that pointer directly, instead of first ensuring a dma-buf exists.

v2: Note that I screwed up the patch ordering, hence why this needed
a Fixes: tag - CI spotted the broken intermediate state.

Fixes: 5eab998741c8 ("drm/vgem: Drop drm_gem_prime_export")
Reviewed-by: Emil Velikov <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vgem/vgem_fence.c

···
 	return &fence->base;
 }
 
-static int attach_dmabuf(struct drm_device *dev,
-			 struct drm_gem_object *obj)
-{
-	struct dma_buf *dmabuf;
-
-	if (obj->dma_buf)
-		return 0;
-
-	dmabuf = dev->driver->gem_prime_export(obj, 0);
-	if (IS_ERR(dmabuf))
-		return PTR_ERR(dmabuf);
-
-	obj->dma_buf = dmabuf;
-	return 0;
-}
-
 /*
  * vgem_fence_attach_ioctl (DRM_IOCTL_VGEM_FENCE_ATTACH):
  *
···
 	if (!obj)
 		return -ENOENT;
 
-	ret = attach_dmabuf(dev, obj);
-	if (ret)
-		goto err;
-
 	fence = vgem_fence_create(vfile, arg->flags);
 	if (!fence) {
 		ret = -ENOMEM;
···
 	}
 
 	/* Check for a conflicting fence */
-	resv = obj->dma_buf->resv;
+	resv = obj->resv;
 	if (!reservation_object_test_signaled_rcu(resv,
 						  arg->flags & VGEM_FENCE_WRITE)) {
 		ret = -EBUSY;