vringh_iov_push_user
EXPORT_SYMBOL(vringh_iov_push_user);
ssize_t vringh_iov_push_user(struct vringh_iov *wiov,
err = vringh_iov_push_user(&wiov, rbuf, rlen);
err = vringh_iov_push_user(&wiov, buf, 5);
assert(vringh_iov_push_user(&wiov, buf, 5) == 0);