mm/page_alloc: add a VM_BUG in __free_one_page() if the zone is uninitialized.

Freeing pages to uninitialized zones is not handled by
__free_one_page(), and should never happen when the code is correct.

Ran into this while writing some code that dynamically onlines extra
zones.

Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Cc: David Hansen <dave@linux.vnet.ibm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -538,6 +538,8 @@ static inline void __free_one_page(struct page *page,
 	unsigned long uninitialized_var(buddy_idx);
 	struct page *buddy;
 
+	VM_BUG_ON(!zone_is_initialized(zone));
+
 	if (unlikely(PageCompound(page)))
 		if (unlikely(destroy_compound_page(page, order)))
 			return;