Ethnic differences in knowledge, attitudes, and practices related to dietary salt intake and association with hypertension in Malaysia: a multi-centre cross-sectional study by Chia, Yook Chin, Ching, Siew Mooi, Chew, Ming Tsuey, Devaraj, Navin Kumar, Oui, Jethro Ee Keat, Lim, Hooi Min, Chew, Bee Nah, Mohamed, Mohazmi, Ooi, Pei Boon, Cheng, Maong Hui, Beh, Hooi Chin, Chung, Felicia Fei-Lei