share_pool: Extract sp_k2u_prepare and sp_k2u_finish

ascend inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4EUVI


CVE: NA

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Refactor sp_k2u to improve its readability.
1. Introduce struct sp_k2u_context to save mapping parameters.
2. Extract sp_k2u_prepare to check input parameters of sp_k2u
   and initialize sp_k2u_context instance.
3. Extract sp_k2u_finish.

Signed-off-by: Tang Yizhou <tangyizhou@huawei.com>
Reviewed-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: Zhou Guanghui <zhouguanghui1@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>