share_pool: Adjust the position of do_mmap checker

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


CVE: NA

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do_mmap checker should be put before MAP_FIXED.

Signed-off-by: Tang Yizhou <tangyizhou@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>