SCSI and ATA drives rescan the drive on opens to catch changes to the
disk. We do it here to so we catch if a drive has been FORMATed or
SANITIZEd with different parameters. We don't use xpt_rescan() since we
don't want to interfere with boot or keep all busses locked (this rescan
won't change the bus, so we don't need the CAM topo lock).
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