Was added in:
  c8b97818
  "Btrfs: Add zlib compression support"
Survive to near time (from 08.10.2008).

Because 'start' checked for zero before branch, so it's safe to remove
that subtraction.
Signed-off-by: NTimofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: NSatoru Takeuchi <satoru.takeuchi@gmail.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>