dccp: do not use ipv6 header for ipv4 flow

When a dual stack dccp listener accepts an ipv4 flow,
it should not attempt to use an ipv6 header or
inet6_iif() helper.

Fixes: 3df80d93

 ("[DCCP]: Introduce DCCPv6")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>