sa_buf_hold
err = sa_buf_hold(os, child_obj, FTAG, &db);
int sa_buf_hold(objset_t *, uint64_t, void *, dmu_buf_t **);
err = sa_buf_hold(zfsvfs->z_os, obj_num, NULL, &db);
err = sa_buf_hold(zfsvfs->z_os, obj_num, NULL, &db);
if ((error = sa_buf_hold(osp, obj, tag, db)) != 0)
VERIFY0(sa_buf_hold(zfsvfs->z_os, obj, NULL, &db));