dma-buf: remove shared fence staging in reservation object

No need for that any more. Just replace the list when there isn't enough
room any more for the additional fence.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.kernel.org/patch/10626143/