scsi: ipr: Use sgl_alloc_order() and sgl_free_order()

Use the sgl_alloc_order() and sgl_free_order() functions instead of open
coding these functions.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Acked-by: Brian King <brking@linux.vnet.ibm.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: linux-scsi@vger.kernel.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>