mm/page_alloc: fix counting of managed_pages

hulk inclusion
category: bugfix
bugzilla: 51887
CVE: NA

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The commit f6366156 (mm/page_alloc.c: clear out zone->lowmem_reserve[]
if the zone is empty) clear out zone->lowmem_reserve[] if zone is empty.
But when zone is not empty and sysctl_lowmem_reserve_ratio[i] is set to zero,
zone_managed_pages(zone) is not counted in the managed_pages either. This is
inconsistent with the description of lowmen_reserve, so fix it.

Fixes: f6366156 ("mm/page_alloc.c: clear out zone->lowmem_reserve[] if the zone is empty")
Reported-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>