UPSTREAM: dma-buf: clarify dma_fence_add_callback documentation

That the caller doesn't need to keep a reference is rather
risky and not defensive at all.

Especially dma_buf_poll got that horrible wrong, so better
remove that sentence and also clarify that the callback
might be called in atomic or interrupt context.

Change-Id: I54fcd09bd3575d6bd1a37d812efc21981776645f
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210901120240.7339-3-christian.koenig@amd.com


(cherry picked from commit 044e55b1)
Bug: 394776841
Bug: 416133632
Signed-off-by: T.J. Mercier <tjmercier@google.com>