[FNT] Decoding algorithm requires k < q/2

[lamphamsy]

For RS FNT, the current decoding algorithm use FFT of length 2*k that should be less than q. Hence, the acceptable k must be smaller than half of q.

Source code: https://github.com/scality/quadiron/blob/master/src/fec_rs_fnt.h#L117

        unsigned len_2k = this->gf->get_code_len_high_compo(2 * this->n_data);
        this->fft_2k = std::make_unique<fft::Radix2<T>>(
            *(this->gf), len_2k, len_2k, this->pkt_size);