mm/dynamic_hugetlb: hold the lock until pages back to hugetlb

hulk inclusion
category: bugfix
bugzilla: 46904 https://gitee.com/openeuler/kernel/issues/I4Y0XO



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Do not release the lock after merging all pages, otherwise some other process
may allocate the pages, and then some pages can't be put back to hugetlb.

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