rmap: always add new vmas at the end

Make sure to always add new VMAs at the end of the list.  This is
important so rmap_walk does not miss a VMA that was created during the
rmap_walk.

The old code got this right most of the time due to luck, but was buggy
when anon_vma_prepare reused a mergeable anon_vma.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -147,7 +147,7 @@ int anon_vma_prepare(struct vm_area_struct *vma)
 			avc->anon_vma = anon_vma;
 			avc->vma = vma;
 			list_add(&avc->same_vma, &vma->anon_vma_chain);
-			list_add(&avc->same_anon_vma, &anon_vma->head);
+			list_add_tail(&avc->same_anon_vma, &anon_vma->head);
 			allocated = NULL;
 			avc = NULL;
 		}