mm/dynamic_hugetlb: add merge page function

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


CVE: NA

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When destroying hpool or alloc huge pages, the pages has been split
may need to be merged to huge pages. Add functions to merge pages in
dhugetlb_pool. The information about split huge pages has been recorded
in hugepage_splitlists and can traverse it to merge huge pages.

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