PR: https://github.com/truenas/middleware/pull/10173

Investigating scalability problems on a large internal system showed that an abnormal amount of time was spent in `psutil.disk_io_counters`. Specifically, it was busy doing `readlines()` on `/proc/diskstats`. This fixes that problem by transposing the same logic to this class but instead of reading the entire file into memory, iterate over it. Generating flamegraph afterwards has shown that this is no longer a problem.