BACKPORT: dma-buf: generalize dma_fence unwrap & merging v3

Introduce a dma_fence_unwrap_merge() macro which allows to unwrap fences
which potentially can be containers as well and then merge them back
together into a flat dma_fence_array.

v2: rename the function, add some more comments about how the wrapper is
    used, move filtering of signaled fences into the unwrap iterator,
    add complex selftest which covers more cases.
v3: fix signaled fence filtering once more

Bug: 379008999
Change-Id: I85a495735b4119fb45d16c465770735f13089d54
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220518135844.3338-5-christian.koenig@amd.com


(cherry picked from commit 245a4a7b)
[TJ: Drop these older dma-fence-unwrap.c changes since we already have
the latest code from our LTS merge.]
Signed-off-by: T.J. Mercier <tjmercier@google.com>