benchmark: fix off-by-one error in fs benchmarks

Fix a off-by-one error that made the benchmarks for asynchronous
functions run `n - 1` times instead of `n` times.

PR-URL: https://github.com/nodejs/node/pull/8338


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>