docs/zh_CN: add vm hmm translation

Translate .../vm/hmm.rst into Chinese.
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/6248f340dd43097d6517239e1f038165b9fef7cd.1650794774.git.siyanteng@loongson.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/vm/index.rst

@@ -28,6 +28,7 @@ TODO:待引用文档集被翻译完毕后请及时修改此处）
    highmem
    ksm
    frontswap
+   hmm
    hwpoison
    hugetlbfs_reserv
    memory-model
@@ -44,7 +45,6 @@ TODO:待引用文档集被翻译完毕后请及时修改此处）
 TODOLIST:
 * arch_pgtable_helpers
 * free_page_reporting
-* hmm
 * hugetlbfs_reserv
 * numa
 * page_migration