mm/sharepool: Add helper for master_list

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



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Add two Helper functions sp_add_group_master and
sp_del_group_master to manipulate master_list.

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