macsec: avoid use-after-free in macsec_handle_frame()

De-referencing skb after call to gro_cells_receive() is not allowed.
We need to fetch skb->len earlier.

Fixes: 5491e7c6 ("macsec: enable GRO and RPS on macsec devices")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

drivers/net/macsec.c

@@ -1077,6 +1077,7 @@ static rx_handler_result_t macsec_handle_frame(struct sk_buff **pskb)
 	struct macsec_rx_sa *rx_sa;
 	struct macsec_rxh_data *rxd;
 	struct macsec_dev *macsec;
+	unsigned int len;
 	sci_t sci;
 	u32 hdr_pn;
 	bool cbit;
@@ -1232,9 +1233,10 @@ static rx_handler_result_t macsec_handle_frame(struct sk_buff **pskb)
 	macsec_rxsc_put(rx_sc);
 
 	skb_orphan(skb);
+	len = skb->len;
 	ret = gro_cells_receive(&macsec->gro_cells, skb);
 	if (ret == NET_RX_SUCCESS)
-		count_rx(dev, skb->len);
+		count_rx(dev, len);
 	else
 		macsec->secy.netdev->stats.rx_dropped++;