memcg: make sysctl registration more extensible

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8QK6Q


CVE: NA

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Move sysctl registration code out of CONFIG_MEMCG_OOM_PRIORITY, to
make the code reused by other features.

Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>