BACKPORT: dma-buf: add dma_fence_unwrap v2

Add a general purpose helper to deep dive into dma_fence_chain/dma_fence_array
structures and iterate over all the fences in them.

This is useful when we need to flatten out all fences in those structures.

v2: some selftests cleanup, improved function naming and documentation

Change-Id: I4d4bd21fd9444523917cc884107f5af13fa5a08f
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220311110244.1245-1-christian.koenig@amd.com


(cherry picked from commit 64a8f92f)
[TJ: Don't bring in dma_resv code]
Bug: 394776841
Signed-off-by: T.J. Mercier <tjmercier@google.com>