- scsi_host_busy_inc isn't used anymore (and uses broken locking rules),
  kill it.
- scsi_host_busy_dec_and_test gets replace by scsi_device_unbusy that
  also cares for sdev->device_busty.
- there's a new helper, scsi_eh_wakeup, shared by scsi_device_unbusy
  and some EH code.