bch_initial_mark_key
bch_initial_mark_key(b->c, b->level, k);
bch_initial_mark_key(b->c, b->level + 1, &b->key);
bch_initial_mark_key(c, c->root->level, k);
bch_initial_mark_key(c, c->root->level + 1, &c->root->key);
void bch_initial_mark_key(struct cache_set *c, int level, struct bkey *k);
bch_initial_mark_key(c, 0, k);