memcg: Add static key for memcg kswapd

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4IMAK?from=project-issue


CVE: NA

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This patch adds a default-false static key to disable memcg kswapd
feature. User can enable by set memcg_kswapd in cmdline.

Signed-off-by: Lu Jialin <lujialin4@huawei.com>
Reviewed-by: weiyang wang <wangweiyang2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>