From NetBSD: (required for last change to scsi_base.c in case we get

From NetBSD: (required for last change to scsi_base.c in case we get
	      "command aborted" status)
Handle cases like the following:
        - controller calls scsi_done() with error XS_TIMEOUT
        - scsi_done() calls sddone()
        - sddone() calls disk_unbusy()
        - scsi_done() calls controller to retry command (missing the
          call to disk_busy())
        - controller calls scsi_done()
        - scsi_done() calls sddone()
        - sddone() calls disk_busy(), which panics because of the imbalance.
Bug noticed by Leo Weppleman, who also suggested this fix; pass an additional
boolean argument ("complete") to the device's "done" routine, with a
value of `0' passed from the previous call to "done", and add an additional
call to "done" when the xfer resources are freed.