mm/sharepool: Implement mg_sp_group_add_task()

ascend inclusion
category: Feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8LNGH



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Add tasks to share pool group. The share memory regions alloced before
are mapped to the new joiner too.

Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>