selftests/mm: check after-split folio orders in split_huge_page_test

selftests/mm: check after-split folio orders in split_huge_page_test

Instead of just checking the existence of PMD folios before and after folio
split tests, use check_folio_orders() to check after-split folio orders.

The split ranges in split_thp_in_pagecache_to_order_at() are changed to
[addr, addr + pagesize) for every pmd_pagesize. It prevents folios within
the range being split multiple times due to debugfs split function always
perform splits with a pagesize step for a given range.

The following tests are not changed:
1. split_pte_mapped_thp: the test already uses kpageflags to check;
2. split_file_backed_thp: no vaddr available.

Link: https://lkml.kernel.org/r/20250818184622.1521620-6-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Donet Tom <donettom@linux.ibm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: wang lian <lianux.mm@gmail.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>