KUALA LUMPUR, June 18 (Reuters) - Societe Generale (SocGen)SOGN.PA has concluded the marketing roadshow for its 1 billion ringgit ($311 million) sale of Islamic bonds in Malaysia and will decide on the size of its first issuance within days.France&aposs third-largest bank by assets plans to sell sukuk with maturities ranging up to 15 years, and has already received approval from Malaysia&aposs cen