The mortgage-backed securities market, which had just started to get comfortable in a low prepayment environment, may be due for a major shock in the weeks ahead as an avalanche of new refinancings in the primary market could be just around the corner. According to a weekly survey by Inside Mortgage Finance, an affiliated newsletter, the average rate quoted on 30-year fixed-rate mortgages fell to 5.40 percent with 1.2 points as of the middle of…