mm, kasan: fix to call kasan_free_pages() after poisoning page

When CONFIG_PAGE_POISONING and CONFIG_KASAN is enabled,
free_pages_prepare()'s codeflow is below.

  1)kmemcheck_free_shadow()
  2)kasan_free_pages()
    - set shadow byte of page is freed
  3)kernel_poison_pages()
  3.1) check access to page is valid or not using kasan
    ---> error occur, kasan think it is invalid access
  3.2) poison page
  4)kernel_map_pages()

So kasan_free_pages() should be called after poisoning the page.

Link: http://lkml.kernel.org/r/1463220405-7455-1-git-send-email-iamyooon@gmail.comSigned-off-by: Nseokhoon.yoon <iamyooon@gmail.com>
Cc: Andrey Ryabinin <a.ryabinin@samsung.com>
Cc: Laura Abbott <labbott@fedoraproject.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -993,7 +993,6 @@ static __always_inline bool free_pages_prepare(struct page *page,
 
 	trace_mm_page_free(page, order);
 	kmemcheck_free_shadow(page, order);
-	kasan_free_pages(page, order);
 
 	/*
 	 * Check tail pages before head page information is cleared to
@@ -1035,6 +1034,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
 	arch_free_page(page, order);
 	kernel_poison_pages(page, 1 << order, 0);
 	kernel_map_pages(page, 1 << order, 0);
+	kasan_free_pages(page, order);
 
 	return true;
 }