mm/vmscan: setup drop_caches_loop_limit in cmdline

hulk inclusion
category: bugfix
bugzilla: 175105
CVE: NA

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When !CONFIG_SYSCTL, drop_caches_loop_limit is invisible.
Add cmdline "drop_caches_loop_limit=" to set it.
This parameter can limit the number of loops per node.

Fixes: ceb5620e ("mm/vmscan: add drop_caches_loop_limit to break loop in drop_slab_node")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>