docs_zh_CN: add active_mm translation

Translatr .../vm/active_mm.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Tang Yizhou <tangyizhou@huawei.com>
Link: https://lore.kernel.org/r/99ba014bdd9550bad57db6c21653cb7314d7c2d8.1641866889.git.siyanteng@loongson.cn


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