fix auditioning on systems with larger disk read chunk sizes.

fix auditioning on systems with larger disk read chunk sizes.

This really brings into focus the question of whether any of the "varifill" code should
really remain. This was probably one of the only remaining places where a partial-fill
operation was done and the code was broken for this case. What we know about disk i/o
streaming doesn't support varifill much at all. Something to think about.