scsi: hisi_sas: allocate slot buffer earlier

Currently we allocate the slot's memory buffer after allocating the DQ
slot.

To aid DQ lockout reduction, and allow slots to be built in parallel,
move this step (which can fail) prior to allocating the slot.

Also a stray spin_unlock_irqrestore() is removed from internal task exec
function.

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>