docs/zh_CN: add core-api genalloc translation

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

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/38d8e13b99152d0a8454db675c1c80fb5c640647.1631846923.git.siyanteng@loongson.cn


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