mbind: add BUG_ON(!vma) in new_vma_page()

new_vma_page() is called only by page migration called from do_mbind(),
where pages to be migrated are queued into a pagelist by
queue_pages_range().  queue_pages_range() confirms that a queued page
belongs to some vma, so !vma case is not supposed to be happen.  This
patch adds BUG_ON() to catch this unexpected case.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Naoya Horiguchi

mm/mempolicy.c

@@ -1196,12 +1196,14 @@ static struct page *new_vma_page(struct page *page, unsigned long private, int *
 			break;
 		vma = vma->vm_next;
 	}
+	/*
+	 * queue_pages_range() confirms that @page belongs to some vma,
+	 * so vma shouldn't be NULL.
+	 */
+	BUG_ON(!vma);
 
 	if (PageHuge(page))
 		return alloc_huge_page_noerr(vma, address, 1);
-	/*
-	 * if !vma, alloc_page_vma() will use task or system default policy
-	 */
 	return alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
 }
 #else