dma-buf: cleanup dma_fence_chain_walk

Use unrcu_pointer() instead of the manual cast.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220518085446.31338-1-christian.koenig@amd.com