scsi: hisi_sas: lock sensitive region in hisi_sas_slot_abort()

When we call hisi_sas_slot_task_free() we should grab the hisi_hba.lock,
as hisi_sas_slot_task_free() accesses common hisi_hba elements.
Function hisi_sas_slot_abort() is missing this, so add it.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Tested-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>