mm/dynamic_hugetlb: fix kabi broken when enable CONFIG_DYNAMIC_HUGETLB on arm64

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6XOIE


CVE: NA

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When enable dynamic hugetlb on arm64, the new member struct dhugetlb_pool*
will be added to mem_cgroup. We need to use a KABI_RESERVE to fix broken
of kabi. The previous struct dhugetlb_pool* is only used on x86_64.

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