mm/hugetlb: fix set_max_huge_pages() when there are surplus pages

mm/hugetlb: fix set_max_huge_pages() when there are surplus pages

In set_max_huge_pages(), min_count should mean the acquired persistent
huge pages, but it contains surplus huge pages.  It will lead to failing
to free free huge pages for a node.

Steps to reproduce:
1) create 5 huge pages in Node 0
2) run a program to use all the huge pages
3) create 5 huge pages in Node 1
4) echo 0 > nr_hugepages for Node 1 to free the huge pages

The result:
        Node 0    Node 1
Total     5         5
Free      0         5
Surp      5         5

With this patch, step 4) destroys the 5 huge pages in Node 1

The result with this patch:
        Node 0    Node 1
Total     5         0
Free      0         0
Surp      5         0

Link: https://lkml.kernel.org/r/20250225141933.3852667-1-tujinjiang@huawei.com
Fixes: 9a30523066cd ("hugetlb: add per node hstate attributes")
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>