memcg: fix kabi broken when enable CONFIG_MEMCG_QOS

hulk inclusion
category: feature
bugzilla: 51827
CVE: NA

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Fix it by moving memcg_priority from struct mem_cgroup to
struct mem_cgroup_extension.

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