hugetlb: ensure we do not reference a surplus page after handing it to buddy

When we free a page via free_huge_page and we detect that we are in surplus
the page will be returned to the buddy.  After this we no longer own the page.

However at the end free_huge_page we clear out our mapping pointer from
page private.  Even where the page is not a surplus we free the page to
the hugepage pool, drop the pool locks and then clear page private.  In
either case the page may have been reallocated.  BAD.

Make sure we clear out page private before we free the page.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Acked-by: Adam Litke <agl@us.ibm.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -120,6 +120,7 @@ static void free_huge_page(struct page *page)
 	struct address_space *mapping;
 
 	mapping = (struct address_space *) page_private(page);
+	set_page_private(page, 0);
 	BUG_ON(page_count(page));
 	INIT_LIST_HEAD(&page->lru);
 
@@ -134,7 +135,6 @@ static void free_huge_page(struct page *page)
 	spin_unlock(&hugetlb_lock);
 	if (mapping)
 		hugetlb_put_quota(mapping, 1);
-	set_page_private(page, 0);
 }
 
 /*