[DCCP] ipv6: dccp_v6_send_response() has a DST leak too.

It was copy&pasted from tcp_v6_send_synack() which has
a DST leak recently fixed by Eric W. Biederman.

So dccp_v6_send_response() needs the same fix too.

Signed-off-by: David S. Miller <davem@davemloft.net>