share_pool: Extract sp_alloc_mmap_populate

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


CVE: NA

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Refactor sp_alloc to improve its readability.

Extract sp_alloc_mmap_populate, which consists of sp_alloc_mmap and
sp_alloc_populate.

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>