ide-disk: add proc_idedisk_read_smart() helper

* Factor out common code from proc_idedisk_read_smart_{thresholds,values}()
  to proc_idedisk_read_smart() helper.

* Rename proc_idedisk_read_smart_thresholds() to proc_idedisk_read_st()
  and proc_idedisk_read_smart_values() to proc_idedisk_read_sv().

There should be no functional changes caused by this patch.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>