mm/dynamic_hugetlb: add migration function

hulk inclusion
category: feature
bugzilla: 46904, https://gitee.com/openeuler/kernel/issues/I4QSHG


CVE: NA

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Sometimes, page merge may failed because some pages are still in use.
Add migration function to enhance the merge function.
This function relies on memory hotremove, so it only works when config
MEMORY_HOTREMOVE is selected.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>