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: Peter Oskolkov <posk@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>