mm/huge_memory: use pfn_to_online_page() in split_huge_pages_all()

NULL pointer dereference is triggered when calling thp split via debugfs
on the system with offlined memory blocks.  With debug option enabled, the
following kernel messages are printed out:

  page:00000000467f4890 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x121c000
  flags: 0x17fffc00000000(node=0|zone=2|lastcpupid=0x1ffff)
  raw: 0017fffc00000000 0000000000000000 dead000000000122 0000000000000000
  raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
  page dumped because: unmovable page
  page:000000007d7ab72e is uninitialized and poisoned
  page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
  ------------[ cut here ]------------
  kernel BUG at include/linux/mm.h:1248!
  invalid opcode: 0000 [raspberrypi#1] PREEMPT SMP PTI
  CPU: 16 PID: 20964 Comm: bash Tainted: G          I        6.0.0-rc3-foll-numa+ raspberrypi#41
  ...
  RIP: 0010:split_huge_pages_write+0xcf4/0xe30

This shows that page_to_nid() in page_zone() is unexpectedly called for an
offlined memmap.

Use pfn_to_online_page() to get struct page in PFN walker.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: f1dd2cd ("mm, memory_hotplug: do not associate hotadded memory to zones until online")      [visible after d0dc12e]
Signed-off-by: Naoya Horiguchi <[email protected]>
Co-developed-by: David Hildenbrand <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: <[email protected]>	[5.10+]
Signed-off-by: Andrew Morton <[email protected]>

Author: nhoriguchi

mm/huge_memory.c

@@ -2894,11 +2894,9 @@ static void split_huge_pages_all(void)
 		max_zone_pfn = zone_end_pfn(zone);
 		for (pfn = zone->zone_start_pfn; pfn < max_zone_pfn; pfn++) {
 			int nr_pages;
-			if (!pfn_valid(pfn))
-				continue;
 
-			page = pfn_to_page(pfn);
-			if (!get_page_unless_zero(page))
+			page = pfn_to_online_page(pfn);
+			if (!page || !get_page_unless_zero(page))
 				continue;
 
 			if (zone != page_zone(page))