mm/sharepool: Refactoring proc file interface similar code

hulk inclusion
category: other
bugzilla: https://gitee.com/openeuler/kernel/issues/I6HRGK



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In spa_overview_show, spg_info_show and spg_overview_show,
there is similar code.

The solution is to extract the difference into the function macro.

Signed-off-by: Xu Qiang <xuqiang36@huawei.com>