dma-buf: make returning the exclusive fence optional

Change reservation_object_get_fences_rcu to make the exclusive fence
pointer optional.

If not specified the exclusive fence is put into the fence array as
well.

This is helpful for a couple of cases where we need all fences in a
single array.

Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180110125341.3618-1-christian.koenig@amd.com