MassHousing, the state’s affordable housing bank, announced a $10.1 million loan closing Monday that will preserve affordable housing at BF Faulkner Tower in Somerville, a 130-apartment community for senior citizens that was constructed in 1978. BF Faulkner Tower’s original subsidized 40-year mortgage was scheduled to be paid off in 2017. When such loans are paid off, affordability restrictions usually end, and owners can convert a property to market-rate rents. Another option is refinancing with MassHousing and agreeing to continue to provide affordable rents. Walnut Hill Tower Associates, which owns the BF Faulkner property, decided to close a new 40-year loan of $10.1 million from MassHousing.