mm/huge_memory: fix folio split check for anon folios in swapcache

Both uniform and non uniform split check missed the check to prevent
splitting anon folios in swapcache to non-zero order.

Splitting anon folios in swapcache to non-zero order can cause data
corruption since swapcache only support PMD order and order-0 entries. 
This can happen when one use split_huge_pages under debugfs to split
anon folios in swapcache.

In-tree callers do not perform such an illegal operation.  Only debugfs
interface could trigger it.  I will put adding a test case on my TODO
list.

Fix the check.

Link: https://lkml.kernel.org/r/20251105162910.752266-1-ziy@nvidia.com
Fixes: 58729c0 ("mm/huge_memory: add buddy allocator like (non-uniform) folio_split()")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reported-by: "David Hildenbrand (Red Hat)" <david@kernel.org>
Closes: https://lore.kernel.org/all/dc0ecc2c-4089-484f-917f-920fdca4c898@kernel.org/
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: x-y-z

mm/huge_memory.c

@@ -3522,7 +3522,8 @@ bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
 		/* order-1 is not supported for anonymous THP. */
 		VM_WARN_ONCE(warns && new_order == 1,
 				"Cannot split to order-1 folio");
-		return new_order != 1;
+		if (new_order == 1)
+			return false;
 	} else if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
 	    !mapping_large_folio_support(folio->mapping)) {
 		/*
@@ -3553,7 +3554,8 @@ bool uniform_split_supported(struct folio *folio, unsigned int new_order,
 	if (folio_test_anon(folio)) {
 		VM_WARN_ONCE(warns && new_order == 1,
 				"Cannot split to order-1 folio");
-		return new_order != 1;
+		if (new_order == 1)
+			return false;
 	} else  if (new_order) {
 		if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
 		    !mapping_large_folio_support(folio->mapping)) {