Fix smartctl exit status success check
According to the smartctl man page: ``` EXIT STATUS The exit statuses of smartctl are defined by a bitmask. If all is well with the disk, the exit status (return value) of smartctl is 0 (all bits turned off). If a problem occurs, or an error, potential error, or fault is detected, then a non-zero status is returned. In this case, the eight different bits in the exit status have the following meanings for ATA disks; some of these values may also be returned for SCSI disks. Bit 0: Command line did not parse. Bit 1: Device open failed, device did not return an IDENTIFY DEVICE structure, or device is in a low-power mode (see '-n' option above). Bit 2: Some SMART or other ATA command to the disk failed, or there was a checksum error in a SMART data structure (see '-b' option above). Bit 3: SMART status check returned "DISK FAILING". Bit 4: We found prefail Attributes <= threshold. Bit 5: SMART status check returned "DISK OK" but we found that some (usage or pre...