mm, hugetlb: increment the number of pages to be reset on HVO

commit 4eeec8c89a0c ("mm: move hugetlb specific things in folio to
page[3]") shifted hugetlb specific stuff, and now mapping overlaps
_hugetlb_cgroup field.

Upon restoring the vmemmap for HVO, only the first two tail pages are
reset, and this causes the check in free_tail_page_prepare() to fail as it
finds an unexpected mapping value in some tails.

Increment the number of pages to be reset to 4 (head + 3 tail pages)

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 4eeec8c89a0c ("mm: move hugetlb specific things in folio to page[3]")
Signed-off-by: Oscar Salvador <[e-mail]>
Suggested-by: David Hildenbrand <[e-mail]>
Reviewed-by: David Hildenbrand <[e-mail]>
Reviewed-by: Muchun Song <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/hugetlb_vmemmap.c

···
  * struct page, the special metadata (e.g. page->flags or page->mapping)
  * cannot copy to the tail struct page structs. The invalid value will be
  * checked in the free_tail_page_prepare(). In order to avoid the message
- * of "corrupted mapping in tail page". We need to reset at least 3 (one
- * head struct page struct and two tail struct page structs) struct page
+ * of "corrupted mapping in tail page". We need to reset at least 4 (one
+ * head struct page struct and three tail struct page structs) struct page
  * structs.
  */
-#define NR_RESET_STRUCT_PAGE		3
+#define NR_RESET_STRUCT_PAGE		4
 
 static inline void reset_struct_pages(struct page *start)
 {