sd_cmd_rw10
int sd_cmd_rw10(struct scsi_generic *, int, u_int64_t, u_int32_t);
xs->cmdlen = sd_cmd_rw10(&xs->cmd, 0, blkno, nwrt); /* XXX */
xs->cmdlen = sd_cmd_rw10(&xs->cmd, read, secno, nsecs);