Revert "drm/gem-shmem: Use dma_buf from GEM object instance"

In the Linux kernel, the following vulnerability has been resolved:

Revert "drm/gem-shmem: Use dma_buf from GEM object instance"

This reverts commit 1a148af06000e545e714fe3210af3d77ff903c11.

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 5307dce878d4 ("drm/gem: Acquire references on GEM handles for framebuffers") and commit f6bfc9afc751 ("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.

v3: - cc stable