mm/sharepool: Add sp_area management code

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



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The sp_area structure specify a memory region used for share_pool.
Add alloc/get/drop operations for sp-area.
The sp_mapping structure manage a set of sp_area.

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