inet_diag: constify raw_lookup() socket argument

Now both raw_v4_match() and raw_v6_match() accept a const socket,
raw_lookup() can do the same to clarify its role.

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>