bch_bset_init_next
void bch_bset_init_next(struct btree_keys *b, struct bset *i, uint64_t magic);
bch_bset_init_next(&b->keys, b->keys.set->data, bset_magic(&b->c->cache->sb));
bch_bset_init_next(&b->keys, write_block(b),
bch_bset_init_next(&b->keys, write_block(b),