scsi: smartpqi: Fix blocks_per_row static checker issue

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>

drivers/scsi/smartpqi/smartpqi_init.c

@@ -2510,6 +2510,8 @@ static int pci_get_aio_common_raid_map_values(struct pqi_ctrl_info *ctrl_info,
 
 	/* Calculate stripe information for the request. */
 	rmd->blocks_per_row = rmd->data_disks_per_row * rmd->strip_size;
+	if (rmd->blocks_per_row == 0) /* Used as a divisor in many calculations */
+		return PQI_RAID_BYPASS_INELIGIBLE;
 #if BITS_PER_LONG == 32
 	tmpdiv = rmd->first_block;
 	do_div(tmpdiv, rmd->blocks_per_row);
@@ -2559,6 +2561,10 @@ static int pqi_calc_aio_r5_or_r6(struct pqi_scsi_dev_raid_map_data *rmd,
 #if BITS_PER_LONG == 32
 	u64 tmpdiv;
 #endif
+
+	if (rmd->blocks_per_row == 0) /* Used as a divisor in many calculations */
+		return PQI_RAID_BYPASS_INELIGIBLE;
+
 	/* RAID 50/60 */
 	/* Verify first and last block are in same RAID group. */
 	rmd->stripesize = rmd->blocks_per_row * rmd->layout_map_count;
@@ -2662,8 +2668,6 @@ static int pqi_calc_aio_r5_or_r6(struct pqi_scsi_dev_raid_map_data *rmd,
 			rmd->q_parity_it_nexus = raid_map->disk_data[index + 1].aio_handle;
 			rmd->xor_mult = raid_map->disk_data[rmd->map_index].xor_mult[1];
 		}
-		if (rmd->blocks_per_row == 0)
-			return PQI_RAID_BYPASS_INELIGIBLE;
 #if BITS_PER_LONG == 32
 		tmpdiv = rmd->first_block;
 		do_div(tmpdiv, rmd->blocks_per_row);