ravb: Fix potential use-after-free in ravb_rx_gbeth()

The skb is delivered to napi_gro_receive() which may free it, after calling this,
dereferencing skb may trigger use-after-free.

Fixes: 1c59eb67 ("ravb: Fillup ravb_rx_gbeth() stub")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20221203092941.10880-1-yuehaibing@huawei.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ethernet/renesas/ravb_main.c

@@ -841,7 +841,7 @@ static bool ravb_rx_gbeth(struct net_device *ndev, int *quota, int q)
 				napi_gro_receive(&priv->napi[q],
 						 priv->rx_1st_skb);
 				stats->rx_packets++;
-				stats->rx_bytes += priv->rx_1st_skb->len;
+				stats->rx_bytes += pkt_len;
 				break;
 			}
 		}