Use span-based file API in io_benchmark.cc

No functionality changes.

Bug: 435317390
Change-Id: I7f99c930fb04d49ccfcad4c286ef197e3109c9c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7423116
Auto-Submit: Jan Keitel <jkeitel@google.com>
Reviewed-by: Benoit Lize <lizeb@chromium.org>
Commit-Queue: Benoit Lize <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1566940}
NOKEYCHECK=True
GitOrigin-RevId: 431768833faf34f4cbcafce172d709cece68496c