dma-buf: finally make dma_resv_excl_fence private v2

Drivers should never touch this directly.

v2: fix rebase clash

Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321135856.1331-10-christian.koenig@amd.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>