Dan Carpenter found a possible divide by 0 issue in the smartpqi driver in
functions pci_get_aio_common_raid_map_values() and pqi_calc_aio_r5_or_r6().
The variable rmd->blocks_per_row is used as a divisor and could be 0.

       Using rmd->blocks_per_row as a divisor without checking
       it for 0 first.

Correct these possible divide by 0 conditions by insuring that
rmd->blocks_per_row is not zero before usage.  The check for non-0 was too
late to prevent a divide by 0 condition.  Add in a comment to explain why
the check for non-zero is necessary. If the member is 0, return
PQI_RAID_BYPASS_INELIGIBLE before any division is performed.

Link: https://lore.kernel.org/linux-scsi/YG%2F5kWHHAr7w5dU5@mwanda/
Link: https://lore.kernel.org/r/161850492435.7302.392780350442938047.stgit@brunhilda
Fixes: 6702d2c4 ("scsi: smartpqi: Add support for RAID5 and RAID6 writes")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Scott Benesh <scott.benesh@microchip.com>
Reviewed-by: Scott Teel <scott.teel@microchip.com>
Reviewed-by: Mike McGowen <mike.mcgowen@microchip.com>
Reviewed-by: Kevin Barnett <kevin.barnett@microchip.com>
Signed-off-by: Don Brace <don.brace@microchip.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>