qa/live: let the dpdk IDS test size its own mempool

The configured mempool of 511 mbufs is split between the worker queues,
which leaves enough mbufs for at most four workers now that a queue also
reserves an RX burst. As the test runs with "threads: auto", it refuses
to start on any host with more cores than a CI runner.

qa/live/dpdk/suricata-null-ids.yaml

@@ -21,7 +21,7 @@ dpdk:
   interfaces:
     - interface: net_null0 # PCIe address of the NIC port
       threads: auto
-      mempool-size: 511
+      mempool-size: auto
       mempool-cache-size: auto
       rx-descriptors: 16
       tx-descriptors: 16