net/virtio: fix memory leak in mergeable Rx

When there is no enough segments for a packet in mergeable
Rx path, we should free the whole mbuf chain instead of just
recycling the last segment.

Fixes: bcac5aa207f8 ("net/virtio: improve batching in mergeable path")
Cc: stable@dpdk.org

Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>

drivers/net/virtio/virtio_rxtx.c

@@ -1736,7 +1736,7 @@ virtio_recv_mergeable_pkts(void *rx_queue,
 		} else {
 			PMD_RX_LOG(ERR,
 					"No enough segments for packet.");
-			virtio_discard_rxbuf(vq, prev);
+			rte_pktmbuf_free(rx_pkts[nb_rx]);
 			rxvq->stats.errors++;
 			break;
 		}