dccp: preserve const qualifier in dccp_sk()

We can change dccp_sk() to propagate its argument const qualifier,
thanks to container_of_const().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>