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 <[email protected]>
Cc: David Hansen <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a3687af..9614aab 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -538,6 +538,8 @@
 	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;