[SCSI] SCSI sd: fix module init/exit error handling

- Properly handle and unwind errors in init_sd().  Fixes leaks on error,
  if class_register() or scsi_register_driver() failed.

- Ensure that exit_sd() execution order is the perfect inverse of
  initialization order.

FIXME:  If some-but-not-all register_blkdev() calls fail, we wind up
calling unregister_blkdev() for block devices we did not register.
This was a pre-existing bug.

Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>