sstable: increment BlockBytes and BlockReadDuration together

During a cache miss, increment the BlockBytes and BlockReadDuration iterator
stats together, immediately after the read. Previously, if an error occurred,
it was possible for the duration to be incremented without updating the bytes
stats.

sstable/reader.go

@@ -661,6 +661,7 @@ func (r *Reader) readBlock(
  int(bh.Length+blockTrailerLen), readDuration.String())
  }
  if stats != nil {
+ stats.BlockBytes += bh.Length
  stats.BlockReadDuration += readDuration
  }
  if err != nil {
@@ -718,9 +719,6 @@ func (r *Reader) readBlock(
  decompressed = transformed
  }
 
- if stats != nil {
- stats.BlockBytes += bh.Length
- }
  if iterStats != nil {
  iterStats.reportStats(bh.Length, 0, readDuration)
  }