btree_bset_last
if (write_block(b) != btree_bset_last(b) &&
BUG_ON(write_block(b) != btree_bset_last(b));
struct bset *i = btree_bset_last(b);
struct bset *i = btree_bset_last(b);
struct bset *i = btree_bset_last(b);