This patch adds support for cdevs to the st driver. The changes are based on
Doug Gilbert's corresponding changes to the sg driver. Using cdevs brings the
following advantanges:
- support for many drives, currently the maximum is set to 128; the minor
  number assignment is explained in the patch to Documentation/scsi/README.st
- the tape devices appear in /sys/cdev/major/st*, as an example here are
  the entries for the first drive (4 modes, rewind and non-rewind
  devices):
  /sys/cdev/major/st0m0   /sys/cdev/major/st0m1   /sys/cdev/major/st0m2
  /sys/cdev/major/st0m3
  /sys/cdev/major/st0m0n  /sys/cdev/major/st0m1n  /sys/cdev/major/st0m2n
  /sys/cdev/major/st0m3n