mm/sharepool: Move spa_num field to sp_group.

hulk inclusion
category: other
bugzilla: https://gitee.com/openeuler/kernel/issues/I6ET9W



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spa_num is not general information, but differentiated information.
It should not be placed in sp_spg_stat.

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