share_pool: Clear VM_SHAREPOOL when drop sp area

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


CVE: NA

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If all the processes in an sp group are exiting, the sp areas are also
freed. We need to clear VM_SHAREPOOL flag for the k2u sp areas,
otherwise __vunmap will fail later.

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