dhugetlb: set hpool to NULL for cont-bit hugepage

hulk inclusion
category: bugfix
bugzilla: 46904, https://gitee.com/openeuler/kernel/issues/I6FCQZ


CVE: NA

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When dynamic hugetlb is enabled, the hpool should be NULL for cont-bit
hugepage, set it.

Fixes: f15774c6 ("dhugetlb: only support 1G/2M hugepage and ARM64_4K_PAGES")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Nanyong Sun <sunnanyong@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>