scsi: scsi_transport_iscsi: Replace use of system_unbound_wq with system_dfl_wq

Currently if a user enqueue a work item using schedule_delayed_work()
the used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistency cannot be addressed without refactoring the
API.

system_unbound_wq should be the default workqueue so as not to enforce
locality constraints for random work whenever it's not required.

Adding system_dfl_wq to encourage its use when unbound work should be
used.

The old system_unbound_wq will be kept for a few release cycles.

Suggested-by: Tejun Heo <[e-mail]>
Signed-off-by: Marco Crivellari <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/scsi_transport_iscsi.c

···
 			list_del_init(&session->sess_list);
 			spin_unlock_irqrestore(&sesslock, flags);
 
-			queue_work(system_unbound_wq, &session->destroy_work);
+			queue_work(system_dfl_wq, &session->destroy_work);
 		}
 		break;
 	case ISCSI_UEVENT_UNBIND_SESSION: