mm/sharepool: Implement mg_sp_id_of_current()

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



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This is used to get the local group id of current process.  When
the process is exiting, the caller of sp_free/sp_unshare must
supply a real spg_id instead of zero. Because the current->mm is NULL
and we cannot access the spg via the mm_struct of current.

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