mm/dynamic_hugetlb: collects resv allocated for dhugetlb_pool

hulk inclusion
category: feature
bugzilla: 46904, https://gitee.com/openeuler/kernel/issues/I4QSHG


CVE: NA

--------------------------------

The dynamic hugetlb feature is based on hugetlb. There is a reserve count in
hugetlb to determine if there were enough free huge pages to satisfy the
requirement while mmap() to avoid SIGBUS at the next page fault time. Add similar
count for dhugetlb_pool to avoid same problem.

References: Documentation/vm/hugetlbfs_reserv.rst

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>