udp6: constify __udp_v6_is_mcast_sock() socket argument

This clarifies __udp_v6_is_mcast_sock() intent.

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>