[ovsp4rt] PrepareFdbTableEntryforV4VxlanTunnel truncates tunnel_id

[ffoulkes]

The DPDK variant of PrepareFdbTableEntryforV4VxlanTunnel encodes the tunnel_id action parameter as a single byte.

• The source field (tnl_info.vni) is uint32_t.
• The tunnel_id param is 24 bits wide.

param->set_param_id(GetParamId(p4info, L2_FWD_TX_TABLE_ACTION_SET_TUNNEL,
                               ACTION_SET_TUNNEL_PARAM_TUNNEL_ID));
// TODO(derek): 8-bit value for 24-bit action parameter.
param->set_value(EncodeByteValue(1, learn_info.tnl_info.vni));

Note that ES2K tunnel IDs are 20 bits, and DPDK tunnel IDs are 24 bits. Consider using compile-time conditionals (#ifdef xxxx_TARGET) to define target-specific bodies for the EncodeTunnelId() function and using the latter instead of EncodeByteValue().