iov_to_buf
ssize_t iov_to_buf(const struct iovec *, size_t, void **);
len = iov_to_buf(iov, iovcnt, &buf);
assert(iov_to_buf(req->vsr_iov_in, req->vsr_niov_in,
bufsize = iov_to_buf(iov, n, &buf);
ext_data_len = iov_to_buf(req->vsr_data_iov_in,