dbuf_add_ref
dbuf_add_ref(db, NULL);
dbuf_add_ref(db, (void *)(uintptr_t)tx->tx_txg);
dbuf_add_ref(dn->dn_dbuf, NULL);
dbuf_add_ref(dn->dn_dbuf, NULL);
dbuf_add_ref(parent, db);
dbuf_add_ref(db, dnh);
dbuf_add_ref(db, child);
void dbuf_add_ref(dmu_buf_impl_t *db, void *tag);