dma-buf: simplify reservation_object_get_fences_rcu a bit

We can add the exclusive fence to the list after making sure we got
a consistent state.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/322034/?series=64786&rev=1