dma-fence: fix dma_fence_get_rcu_safe v2

dma-fence: fix dma_fence_get_rcu_safe v2

When dma_fence_get_rcu() fails to acquire a reference it doesn't necessary
mean that there is no fence at all.

It usually mean that the fence was replaced by a new one and in this situation
we certainly want to have the new one as result and *NOT* NULL.

v2: Keep extra check after dma_fence_get_rcu().

Signed-off-by: Christian König <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Sumit Semwal <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Reviewed-by: Maarten Lankhorst <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]