Add Chinese translation for vm/ksm.rst

Translate Documentation/vm/ksm.rst into Chinese.
Update Documentation/translations/zh_CN/vm/index.rst.

Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/ceb82d6458cd79bc3b7060199db0c3518adc3b8b.1646899056.git.siyanteng@loongson.cn


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