split_iov
struct iovec *split_iov(struct iovec *, size_t *, size_t, size_t *);
req->vsr_data_iov_out = split_iov(req->vsr_iov_out, &req->vsr_niov_out,
req->vsr_data_iov_in = split_iov(req->vsr_iov_in, &req->vsr_niov_in,