mm/sharepool: Update sp_mapping structure

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EORS


CVE: NA

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1. Add a list for sp_mapping to record all the sp_groups attached to it.
2. Initialize the sp_mapping for local_group when it is created. So when
   we add a task to a group, we should merge the dvpp mapping of the
   local group.
3. Every two groups can be merged if and only if at least one of them is
   empty. Then the empty mapping would be dropped and another mapping
   would be attached to the two groups. This need to traverse all the
   groups attached to the mapping.
4. A mapping is considered empty when no spa is allocated from it and
   its address space is default.

Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>