datapagemap_create() and datapagemap_destroy() were declared extern,
but they don't actually exist anywhere. Per YUriy Zhuravlev and
Michael Paquier.
typedef struct datapagemap datapagemap_t;
typedef struct datapagemap_iterator datapagemap_iterator_t;
-extern datapagemap_t *datapagemap_create(void);
-extern void datapagemap_destroy(datapagemap_t *map);
extern void datapagemap_add(datapagemap_t *map, BlockNumber blkno);
extern datapagemap_iterator_t *datapagemap_iterate(datapagemap_t *map);
extern bool datapagemap_next(datapagemap_iterator_t *iter, BlockNumber *blkno);