mctp i2c: Fix potential use-after-free

The skb is handed off to netif_rx() which may free it.
Found by Smatch.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/mctp/mctp-i2c.c

@@ -338,7 +338,7 @@ static int mctp_i2c_recv(struct mctp_i2c_dev *midev)
 
 	if (status == NET_RX_SUCCESS) {
 		ndev->stats.rx_packets++;
-		ndev->stats.rx_bytes += skb->len;
+		ndev->stats.rx_bytes += recvlen;
 	} else {
 		ndev->stats.rx_dropped++;
 	}