mm/memory.c: fix clear_gigantic_page_chunk

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5323R


CVE: NA

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Patch "mm: parallelize clear_gigantic_page" put mem_map_next() after
clear_user_highpage but not modify the parameter i. So it's actually the
previous page being cleared in each loop. As a result, the last page has
not been cleared. Fix it by put mem_map_next() back to the original
position.

Fixes: ae0cd4d4 ("mm: parallelize clear_gigantic_page")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Wei Li <liwei391@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>