memcg: Add sysctl memcg_qos_enable

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8PXX8


CVE: NA

--------------------------------------

This patch adds sysctl memcg_qos_enable to enable/disable
memcg oom 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>
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>