blst_meta_alloc
static blist_blkno_t blst_meta_alloc(blmeta_t *scan, blist_blkno_t blk,
blk = blst_meta_alloc(bl->bl_root, 0, count, bl->bl_radix, bl->bl_skip);
r = blst_meta_alloc(&scan[i], blk, count, radix, next_skip - 1);