share_pool: add mm address check when access the process's sp_group file

ascend inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4JICC


CVE: NA

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When we access the process's sp_group file and the precess is a kernel
process, it's task_struct->mm value will be 0, so we must check it, and
make sure the process is not a kernel process.

v1->v2: The path of a process as a kernel process is not often triggered,
so add a unlikely function to accelerate execytion.

Signed-off-by: Zhang Jian <zhangjian210@huawei.com>
Reviewed-by: Ding Tianhong <dingtianhong@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>