Propagate DPDK mbuf RX timestamp to FreeBSD mbuf rcv_tstmp

[tarun-t]

ff_veth_input() in ff_dpdk_if.c never copies mbuf->timestamp from the DPDK mbuf to the FreeBSD mbuf's m_pkthdr.rcv_tstmp. The FreeBSD side already has the full
SO_TIMESTAMP → recvmsg() → SCM_TIMESTAMP path wired up — the timestamp just gets dropped at the DPDK→FreeBSD boundary.

The fix is ~10 lines across ff_veth.c, ff_veth.h, ff_dpdk_if.c:

1. Add ff_mbuf_set_timestamp() (same pattern as ff_mbuf_set_vlan_info()):

void ff_mbuf_set_timestamp(void *hdr, uint64_t timestamp) {
    struct mbuf *m = (struct mbuf *)hdr;
    m->m_pkthdr.rcv_tstmp = timestamp;
    m->m_flags |= M_TSTMP | M_TSTMP_HPREC;
}

2. Enable RTE_ETH_RX_OFFLOAD_TIMESTAMP on the port when the NIC supports it.
3. Call the helper in ff_veth_input() when RTE_MBUF_F_RX_IEEE1588_TMST is set:

if (pkt->ol_flags & RTE_MBUF_F_RX_IEEE1588_TMST)
     ff_mbuf_set_timestamp(hdr, pkt->timestamp);

When HW timestamps aren't available the flag is never set, so nothing changes — fully backward compatible.

Happy to raise a PR if you're interested.