mm/sharepool: Allow share THP to kernel

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5DS9S


CVE: NA

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This is not used for THP but the user page table is just like THP. The
user alloc hugepages via a special driver and its vma is not marked with
VM_HUGETLB. This commit allow to share those vma to kernel.

Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>