scsi: iscsi: Use system_unbound_wq for destroy_work

Use the system_unbound_wq for async session destruction. We don't need a
dedicated workqueue for async session destruction because:

1. perf does not seem to be an issue since we only allow 1 active work.

2. it does not have deps with other system works and we can run them in
parallel with each other.

Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Lee Duncan <[e-mail]>
Signed-off-by: Mike Christie <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/scsi_transport_iscsi.c

···
 static atomic_t iscsi_session_nr; /* sysfs session id for next new session */
 static struct workqueue_struct *iscsi_eh_timer_workq;
 
-static struct workqueue_struct *iscsi_destroy_workq;
-
 static DEFINE_IDA(iscsi_sess_ida);
 /*
  * list of registered transports and lock that must
···
 			list_del_init(&session->sess_list);
 			spin_unlock_irqrestore(&sesslock, flags);
 
-			queue_work(iscsi_destroy_workq, &session->destroy_work);
+			queue_work(system_unbound_wq, &session->destroy_work);
 		}
 		break;
 	case ISCSI_UEVENT_UNBIND_SESSION:
···
 		goto release_nls;
 	}
 
-	iscsi_destroy_workq = alloc_workqueue("%s",
-			WQ_SYSFS | __WQ_LEGACY | WQ_MEM_RECLAIM | WQ_UNBOUND,
-			1, "iscsi_destroy");
-	if (!iscsi_destroy_workq) {
-		err = -ENOMEM;
-		goto destroy_wq;
-	}
-
 	return 0;
 
-destroy_wq:
-	destroy_workqueue(iscsi_eh_timer_workq);
 release_nls:
 	netlink_kernel_release(nls);
 unregister_flashnode_bus:
···
 
 static void __exit iscsi_transport_exit(void)
 {
-	destroy_workqueue(iscsi_destroy_workq);
 	destroy_workqueue(iscsi_eh_timer_workq);
 	netlink_kernel_release(nls);
 	bus_unregister(&iscsi_flashnode_bus);