drm/gem: Internally test import_attach for imported objects

Test struct drm_gem_object.import_attach to detect imported objects.

During object clenanup, the dma_buf field might be NULL. Testing it in
an object's free callback then incorrectly does a cleanup as for native
objects. Happens for calls to drm_mode_destroy_dumb_ioctl() that
clears the dma_buf field in drm_gem_object_exported_dma_buf_free().

v3:
- only test for import_attach (Boris)
v2:
- use import_attach.dmabuf instead of dma_buf (Christian)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: b57aa47 ("drm/gem: Test for imported GEM buffers with helper")
Reported-by: Andy Yan <andyshrk@163.com>
Closes: https://lore.kernel.org/dri-devel/38d09d34.4354.196379aa560.Coremail.andyshrk@163.com/
Tested-by: Andy Yan <andyshrk@163.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Anusha Srivatsa <asrivats@redhat.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20250416065820.26076-1-tzimmermann@suse.de

include/drm/drm_gem.h

@@ -585,8 +585,7 @@ static inline bool drm_gem_object_is_shared_for_memory_stats(struct drm_gem_obje
  */
 static inline bool drm_gem_is_imported(const struct drm_gem_object *obj)
 {
-	/* The dma-buf's priv field points to the original GEM object. */
-	return obj->dma_buf && (obj->dma_buf->priv != obj);
+	return !!obj->import_attach;
 }
 
 #ifdef CONFIG_LOCKDEP