[NET]: Nicer WARN_ON in netstat_show

The

        if (statement)
                WARN_ON(1);

looks much better as

        WARN_ON(statement);

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>