zcache iostat -l buckets start at 1us (openzfs#824)

In `zcache iostat -l`, the buckets are labeled starting at 1024ns (and
doubling from there), when in reality the first bucket is 1000ns (1us
exactly).

cmd/zfs_object_agent/zcache/src/iostat.rs

@@ -193,7 +193,7 @@ impl IoStatDisplay {
                 for j in 0..LatencyHistogram::BUCKETS {
                     // First display each bucket name
                     // Use ending range of bucket for latency (first bucket is 1us)
-                    let nice_value = nice_number_time(Duration::from_nanos((1024 << j) - 1));
+                    let nice_value = nice_number_time(Duration::from_micros(1u64 << j));
                     write_stdout!("{:>1$} ", nice_value, self.max_name_length);
 
                     // Then display bucket values for each disk io type (total of 5)