docs/zh_CN: add core api kref translation

Translate Documentation/core-api/kref.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/20210923095559.983679-1-siyanteng@loongson.cn


Signed-off-by: Jonathan Corbet <corbet@lwn.net>