dma-buf: return only unsignaled fences in dma_fence_unwrap_for_each v3

dma_fence_chain containers cleanup signaled fences automatically, so
filter those out from arrays as well.

v2: fix missing walk over the array
v3: massively simplify the patch and actually update the description.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220518135844.3338-4-christian.koenig@amd.com