mm/dynamic_hugetlb: only support to merge 2M dynamicly

hulk inclusion
category: bugfix
bugzilla: 46904 https://gitee.com/openeuler/kernel/issues/I4Y0XO



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

We do not support dynamic combination of 1G hugepages dynamicly as this can
result in a significant performance loss. We suggest to configure the number of
hugepages immediately after creating a dynamic hugetlb pool rather than modify
them dynamicly while some processes are runing.

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