Review mailbox of GGU

[AltraMayor]

Some deployments require increasing the parameter lua/ggu.lua:mailbox_max_entries_exp to handle attacks with a very large number of flows. However, the packets sitting on GGU's mailbox are not accounted for in net_conf->back.total_pkt_burst at ggu/main.c:run_ggu(), and this might limit the number of preallocated packets available to the back interface at GK instances.

For net_conf->back.total_pkt_burst to account for the packets sitting on GGU's mailbox, a new GGU parameter is needed because only accounting for the maximum number of entries in GGU's mailbox ignores that each entry can hold multiple packets. A solution is to add the parameter mailbox_pkts_per_entry to struct ggu_config with the default value max_pkt_burst / 2 set in lua/ggu.lua. This parameter can even be set to zero to save memory when the back interface can implement the filters that the GGU needs in hardware.

Finally, ggu/main.c:process_single_packet() should not release the packets, but its callers should release those packets in bulk (see rte_pktmbuf_free_bulk()) to reduce the cost of releasing packets. An option is to add process_packets() to encapsulate this pattern.