scsi: hisi_sas: release SMP slot in lldd_abort_task

When an SMP task timeouts, it will call lldd_abort_task to release the
associated slot, and then will release the sas_task.

Currently in lldd_abort_task, if we fail to internally abort IO, then
the slot of SMP IO is not released, but sas_task will still be later
released, so the slot's sas_task is NULL, which will cause NULL pointer
when hisi_sas_slot_task_free happens later.

To resolve, check the return value of internal abort, and release the
slot if it failed.

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>