Potential OOB ring access in netmap_bwrap_register for NICs with asymmetric TX/RX ring counts

Summary

netmap_bwrap_register(:1699-1706): ring cross-link iterates na->num_tx_rings accessing hwna->tx_rings[i]. But na->num_tx_rings=hwna->num_rx_rings (bwrap_attach:1866). If NIC num_rx_rings>num_tx_rings: accesses hwna->tx_rings past its allocation. Most NICs symmetric so unlikely; malicious/buggy driver config could exploit.