etmem_scan: add pte_hole callback

euleros inclusion
category: feature
feature: etmem
bugzilla: https://gitee.com/openeuler/kernel/issues/I4OODH?from=project-issue


CVE: NA

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add a callback in pte_hole during walk_page_range for user to scan page
without page table.

Signed-off-by: Kemeng Shi <shikemeng@huawei.com>
Reviewed-by: louhongxiang <louhongxiang@huawei.com>
Reviewed-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>