mm/sharepool: Implement mg_sp_free()

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



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Free the user share memory alloced by sp_alloc(). Note that this would
unmap the memory for all the processes in the share pool group.

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