block: record new size in bdrv_dirty_bitmap_truncate

ce1ffea8 neglected to update the BdrvDirtyBitmap structure
itself for internal consistency. It's currently not an issue,
but for migration and persistence series this will cause headaches.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -3224,6 +3224,7 @@ static void bdrv_dirty_bitmap_truncate(BlockDriverState *bs)
             continue;
         }
         hbitmap_truncate(bitmap->bitmap, size);
+        bitmap->size = size;
     }
 }