raw: preserve const qualifier in raw_sk()

We can change raw_sk() to propagate const qualifier of its argument,
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>