dma-buf: deprecate DMABUF_SYSFS_STATS

Add a warning that this UAPI wasn't such a good idea and shouldn't be
used by anybody.

That should give us a better chance to remove it at some point and
prevents others from running into the same issues.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220617085702.4298-1-christian.koenig@amd.com