bch_bbio_alloc
struct bio *bch_bbio_alloc(struct cache_set *c);
bio = bch_bbio_alloc(b->c);
b->bio = bch_bbio_alloc(b->c);
bio = bch_bbio_alloc(b->c);
struct bio *bio = bch_bbio_alloc(c);
struct bio *bio = bch_bbio_alloc(ca->set);