mm: page_alloc: validate buddy before check its migratetype.

Whenever a buddy page is found, page_is_buddy() should be called to
check its validity.  Add the missing check during pageblock merge check.

Fixes: 1dd214b8 ("mm: page_alloc: avoid merging non-fallbackable pageblocks with others")
Link: https://lore.kernel.org/all/20220330154208.71aca532@gandalf.local.home/Reported-and-tested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -1108,6 +1108,9 @@ static inline void __free_one_page(struct page *page,
 
 		buddy_pfn = __find_buddy_pfn(pfn, order);
 		buddy = page + (buddy_pfn - pfn);
+
+		if (!page_is_buddy(page, buddy, order))
+			goto done_merging;
 		buddy_mt = get_pageblock_migratetype(buddy);
 
 		if (migratetype != buddy_mt