isci: atomic device lookup and reference counting

We have unsafe references to remote devices that are notified to
disappear at lldd_dev_gone.  In order to clean this up we need a single
canonical source for device lookups and stable references once a lookup
succeeds.  Towards that end guarantee that domain_device.lldd_dev is
NULL as soon as we start the process of stopping a device.  Any code
path that wants to safely lookup a remote device must do so through
task->dev->lldd_dev (isci_lookup_device()).

For in-flight references outside of scic_lock we need reference counting
to ensure that the device is not recycled before we are done with it.
Simplify device back references to just scic_sds_request.target_device
which is now the only permissible internal reference that is maintained
relative to the reference count.

There were two occasions where we wanted new i/o's to be treated as
SAS_TASK_UNDELIVERED but where the domain_dev->lldd_dev link is still
intact.  Introduce a 'gone' flag to prevent i/o while waiting for libsas
to take action on the port down event.

One 'core' leftover is that we currently call
scic_remote_device_destruct() from isci_remote_device_deconstruct()
which is called when the 'core' says the device is stopped.  It would be
more natural for the final put to trigger
isci_remote_device_deconstruct() but this implementation is deferred as
it requires other changes.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/scsi/isci/host.c

@@ -1327,8 +1327,8 @@ void isci_host_deinit(struct isci_host *ihost)
 		struct isci_remote_device *idev, *d;
 
 		list_for_each_entry_safe(idev, d, &iport->remote_dev_list, node) {
-			isci_remote_device_change_state(idev, isci_stopping);
-			isci_remote_device_stop(ihost, idev);
+			if (test_bit(IDEV_ALLOCATED, &idev->flags))
+				isci_remote_device_stop(ihost, idev);
 		}
 	}