MBS-8024: don't set the length of pregap tracks in FixTrackLength.
Also don't screw up data tracks with FixTrackLength.
Merge remote-tracking branch 'ian/mbs-8024'

* ian/mbs-8024:
  Also don't screw up data tracks with FixTrackLength.
  MBS-8024: don't set the length of pregap tracks in FixTrackLength.