The `run_once()` name is just a historical accident. Furthermore, it now
started to appear elsewhere as well, so let's just call it IO::step() as we
should have from the beginning.
Fix brekage from first merging commit d959319b ("Merge 'Use u64 for file
offsets in I/O and calculate such offsets in u64' from Preston Thorpe")
and then commit 6591b66c ("Merge 'Simulate I/O in memory' from Pedro
Muniz"), which was unaware of the changes.
