mm/dynamic_pool: migrate used pages before promote to huge page

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


CVE: NA

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When merge 4K subpages to 2M size, some subpages may be in used.
Call do_migrate_range() to free these pages.

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