MINOR: quic: Make use of the last cbuf API when initializing TX ring buffers

Initialize the circular buffer internal buffer from a specific pool for TX ring
buffers named "pool_head_quic_tx_ring".
diff --git a/src/proto_quic.c b/src/proto_quic.c
index 1e2ca57..257a492 100644
--- a/src/proto_quic.c
+++ b/src/proto_quic.c
@@ -537,20 +537,29 @@
 
 	MT_LIST_INIT(&l->rx.tx_qrings);
 	for (i = 0; i < global.nbthread; i++) {
+		unsigned char *buf;
 		struct qring *qr = &l->rx.qrings[i];
 
-		qr->cbuf = cbuf_new();
-		if (!qr->cbuf)
+		buf = pool_alloc(pool_head_quic_tx_ring);
+		if (!buf)
 			goto err;
 
+		qr->cbuf = cbuf_new(buf, QUIC_TX_RING_BUFSZ);
+		if (!qr->cbuf) {
+			pool_free(pool_head_quic_tx_ring, buf);
+			goto err;
+		}
+
 		MT_LIST_APPEND(&l->rx.tx_qrings, &qr->mt_list);
 	}
 
 	return 1;
 
  err:
-	while ((qr = MT_LIST_POP(&l->rx.tx_qrings, typeof(qr), mt_list)))
+	while ((qr = MT_LIST_POP(&l->rx.tx_qrings, typeof(qr), mt_list))) {
+		pool_free(pool_head_quic_tx_ring, qr->cbuf->buf);
 		cbuf_free(qr->cbuf);
+	}
 	free(l->rx.qrings);
 	return 0;
 }