scsi: hisi_sas: Avoid accessing to SSP task for SMP I/Os

hisi_sas_slot_task_free() attempts to dereference SSP task for non-ATA
tasks. If the task is SMP, the code may reference the wrong structure
although this may not cause any problems.

To avoid this, only access to SSP task when slot->n_elem_dif is not 0 which
indicates this is an SSP task.

Link: https://lore.kernel.org/r/1598958790-232272-2-git-send-email-john.garry@huawei.com


Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>