xfrm: dont assume rcu_read_lock in xfrm_output_one()

ip_local_out() is called with rcu_read_lock() held from ip_queue_xmit()
but not from other call sites.
Reported-and-bisected-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/xfrm/xfrm_output.c

@@ -101,7 +101,7 @@ static int xfrm_output_one(struct sk_buff *skb, int err)
 			err = -EHOSTUNREACH;
 			goto error_nolock;
 		}
-		skb_dst_set_noref(skb, dst);
+		skb_dst_set(skb, dst_clone(dst));
 		x = dst->xfrm;
 	} while (x && !(x->outer_mode->flags & XFRM_MODE_FLAG_TUNNEL));