scsi: smartpqi: Refactor scatterlist code

Factor out code common to all scatter-gather list building to prepare for
new AIO functionality. AIO (Accelerated I/O) requests go directly to disk

No functional changes.

Link: https://lore.kernel.org/r/161549372147.25025.9706613054649682229.stgit@brunhilda


Reviewed-by: Scott Benesh <scott.benesh@microchip.com>
Reviewed-by: Mike McGowen <mike.mcgowen@microchip.com>
Reviewed-by: Scott Teel <scott.teel@microchip.com>
Reviewed-by: Kevin Barnett <kevin.barnett@microchip.com>
Reviewed-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Don Brace <don.brace@microchip.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>