mm/dynamic_hugetlb: fix clear PagePool without lock protection

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I66OCA


CVE: NA

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Since free_pages_prepare() will clear the PagePool without lock in
free_page_to_dhugetlb_pool() and free_page_list_to_dhugetlb_pool(),
it is unreliable to check whether a page is freed by PagePool in
hpool_merge_page().
Move free_pages_prepare() after ClearPagePool(), which can guarantee
all allocated page has PagePool flag.

Fixes: 71197c63 ("mm/dynamic_hugetlb: free pages to dhugetlb_pool")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Tong Tiangen <tongtiangen@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>