mm/page_alloc: correct start page when guard page debug is enabled

When guard page debug is enabled and set_page_guard returns success, we
miss to forward page to point to start of next split range and we will do
split unexpectedly in page range without target page.  Move start page
update before set_page_guard to fix this.

As we split to wrong target page, then splited pages are not able to merge
back to original order when target page is put back and splited pages
except target page is not usable.  To be specific:

Consider target page is the third page in buddy page with order 2.
| buddy-2 | Page | Target | Page |

After break down to target page, we will only set first page to Guard
because of bug.
| Guard   | Page | Target | Page |

When we try put_page_back_buddy with target page, the buddy page of target
if neither guard nor buddy, Then it's not able to construct original page
with order 2
| Guard | Page | buddy-0 | Page |

All pages except target page is not in free list and is not usable.

Link: https://lkml.kernel.org/r/20230927094401.68205-1-shikemeng@huaweicloud.com
Fixes: 06be6ff ("mm,hwpoison: rework soft offline for free pages")
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Kemeng Shi

mm/page_alloc.c

@@ -6475,14 +6475,14 @@ static void break_down_buddy_pages(struct zone *zone, struct page *page,
 			next_page = page;
 			current_buddy = page + size;
 		}
+		page = next_page;
 
 		if (set_page_guard(zone, current_buddy, high, migratetype))
 			continue;
 
 		if (current_buddy != target) {
 			add_to_free_list(current_buddy, zone, high, migratetype);
 			set_buddy_order(current_buddy, high);
-			page = next_page;
 		}
 	}
 }