net: fix double-free in bpf_lwt_xmit_reroute

dst_output() frees skb when it fails (see, for example,
ip_finish_output2), so it must not be freed in this case.

Fixes: 3bd0b152 ("bpf: add handling of BPF_LWT_REROUTE to lwt_bpf.c")
Signed-off-by: NPeter Oskolkov <posk@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/lwt_bpf.c

@@ -260,7 +260,7 @@ static int bpf_lwt_xmit_reroute(struct sk_buff *skb)
 
 	err = dst_output(dev_net(skb_dst(skb)->dev), skb->sk, skb);
 	if (unlikely(err))
-		goto err;
+		return err;
 
 	/* ip[6]_finish_output2 understand LWTUNNEL_XMIT_DONE */
 	return LWTUNNEL_XMIT_DONE;