af_packet: preserve const qualifier in pkt_sk()

We can change pkt_sk() to propagate const qualifier of its argument,
thanks to container_of_const()

This should avoid some potential errors caused by accidental
(const -> not_const) promotion.

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