memcg: Add static key for memcg priority

hulk inclusion
category: feature
bugzilla: 51827
CVE: NA

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This patch adds a default-false static key to disable memcg priority
feature. If you want to enable it by writing 1:

echo 1 > /proc/sys/vm/memcg_qos_enable

Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>