macsec: fix use-after-free of skb during RX

Fix use-after-free of skb when rx_handler returns RX_HANDLER_PASS.
Signed-off-by: Andreas Steinmetz <ast@domdv.de>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macsec.c

@@ -1099,10 +1099,9 @@ static rx_handler_result_t macsec_handle_frame(struct sk_buff **pskb)
 	}
 
 	skb = skb_unshare(skb, GFP_ATOMIC);
-	if (!skb) {
-		*pskb = NULL;
+	*pskb = skb;
+	if (!skb)
 		return RX_HANDLER_CONSUMED;
-	}
 
 	pulled_sci = pskb_may_pull(skb, macsec_extra_len(true));
 	if (!pulled_sci) {