mm: Fix return val in khugepaged_scan_pmd()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4SK3S


CVE: NA

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The khugepaged_scan_pmd() should return 0 not SCAN_ALLOC_HUGE_PAGE_FAIL,
or mmap_sem will not be released in khugepaged_scan_mm_slot().

Fixes: 1845e7ad ("mm: Add reliable memory use limit for user tasks")
Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Reviewed-by: Wei Li <liwei391@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>