dmz_first_valid_block
ret = dmz_first_valid_block(zmd, from_zone, &chunk_block);
ret = dmz_first_valid_block(zmd, src_zone, &block);
ret = dmz_first_valid_block(zmd, bzone, &chunk_block);
int dmz_first_valid_block(struct dmz_metadata *zmd, struct dm_zone *zone,