A fifty-year-old man presents to his practitioner complaining that he often feels as if the room is spinning when he gets up from a recumbent position or turns his head. He has not lost consciousness and has had no chest pain. He has no cardiac history and a recent treadmill test showed no abnormalities. On examination, the sensation can be produced by rapidly turning the head. It can be reproduced many times, but it eventually ceases. Nystagmus is elicited. Hearing is normal. Which of the following is the most likely diagnosis?