dma-buf: drop seq count based update

This should be possible now since we don't have the distinction
between exclusive and shared fences any more.

The only possible pitfall is that a dma_fence would be reused during the
RCU grace period, but even that could be handled with a single extra check.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220407085946.744568-15-christian.koenig@amd.com