mm/dynamic_pool: create dpool by dhugetlb.nr_pages interface

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8S9BY


CVE: NA

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With new interface "dhugetlb.nr_pages", the user can create a dpool in
memcg.

To enable dpool, "dynamic_hugetlb=on" should be added to kernel parameters.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>