mm: sharepool: add static modifier to find_spg_node_by_spg()

hulk inclusion
category: cleanup
bugzilla: https://gitee.com/openeuler/kernel/issues/I6D0N5



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Add static modifier to find_spg_node_by_spg(), for this function is
designed as an internal helper function, should only be used in
share_pool.c scope.
Also add static modifier to spa_overview_show and spg_overview_show.

Signed-off-by: Guo Mengqi <guomengqi3@huawei.com>