Documentation: fix a hugetlbfs reservation statement

Documentation: fix a hugetlbfs reservation statement

Documentation/mm/hugetlbfs_reserv.rst has
	if (resv_needed <= (resv_huge_pages - free_huge_pages))
		resv_huge_pages += resv_needed;
which describes this code in gather_surplus_pages()
	needed = (h->resv_huge_pages + delta) - h->free_huge_pages;
	if (needed <= 0) {
		h->resv_huge_pages += delta;
		return 0;
	}
which means if there are enough free hugepages to account for the new
reservation, simply update the global reservation count without
further action.

But the description is backwards, it should be
	if (resv_needed <= (free_huge_pages - resv_huge_pages))
instead.

Link: https://lkml.kernel.org/r/20260302201015.1824798-1-jane.chu@oracle.com
Fixes: 70bc0dc578b3 ("Documentation: vm, add hugetlbfs reservation overview")
Signed-off-by: Jane Chu <jane.chu@oracle.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>