mm: Fallback to non-mirrored region below low watermark

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


CVE: NA

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Commit 45bd608e ("mm: Introduce watermark check for memory reliable")
introduce watermark to reserve memory in mirrored region for kernel usage.
But this value does not interact well with kswapd and this will lead to
fallback to non-mirrored region without release pagecache in mirrored
region.

With this patch, watermark is set to zero to avoid this problem.
Memory allocation with ___GFP_RELIABILITY will fallback to non-mirrored
region if zone's low watermark is reached and kswapd will be awakened at
this time.

Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>