scsi:libsas: fix oops caused by assigning a freed task to ->lldd_task

A freed task has been assigned to ->lldd_task when lldd_execute_task()
failed in sas_ata_qc_issue(), and access of ->lldd_task will cause an
oops:

Call trace:
[<ffffffc000641f64>] sas_ata_post_internal+0x6c/0x150
[<ffffffc0006c0d64>] ata_exec_internal_sg+0x32c/0x588
[<ffffffc0006c1048>] ata_exec_internal+0x88/0xe8
[<ffffffc0006c13b4>] ata_dev_read_id+0x204/0x5e0
[<ffffffc0006c17f0>] ata_dev_reread_id+0x60/0xc8
[<ffffffc0006c3098>] ata_dev_revalidate+0x88/0x1e0
[<ffffffc0006cf828>] ata_eh_recover+0xcf8/0x13a8
[<ffffffc0006d075c>] ata_do_eh+0x5c/0xe0
[<ffffffc0006d0828>] ata_std_error_handler+0x48/0x98
[<ffffffc0006d042c>] ata_scsi_port_error_handler+0x474/0x658
[<ffffffc000641b78>] async_sas_ata_eh+0x50/0x80
[<ffffffc0000ca664>] async_run_entry_fn+0x64/0x180
[<ffffffc0000c085c>] process_one_work+0x164/0x438
[<ffffffc0000c0c74>] worker_thread+0x144/0x4b0
[<ffffffc0000c70fc>] kthread+0xfc/0x110

Fix this by reassigning NULL to ->lldd_task in error path.

Signed-off-by: Wei Fang <fangwei1@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>