firewire: fw-sbp2: fix refcounting

Since patch "fw-sbp2: use an own workqueue (fix system responsiveness)"
increased parallelism between fw-sbp2 and fw-core, it was possible that
fw-sbp2 didn't release the SCSI device when the FireWire device was
disconnected.

This happened if sbp2_update() ran during sbp2_login(), because a bus
reset occurred during sbp2_login().  The sbp2_login() work would [try
to] reschedule itself because it failed due to the bus reset, and it
would _not_ drop its reference on the target.  However, sbp2_update()
would schedule sbp2_login() too before sbp2_login() rescheduled itself
and hence sbp2_update() would take an additional reference.  And then
we would have one reference too many.

The fix is to _always_ drop the reference when leaving the sbp2_login()
work.  If the sbp2_login() work reschedules itself, it takes a
reference, but only if it wasn't already rescheduled by sbp2_update().

Ditto in the sbp2_reconnect() work.

The resulting code is actually simpler than before:  We _always_ take
a reference when successfully scheduling work.  And we _always_ drop
a reference when leaving a workqueue job.  No exceptions.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/fw-sbp2.c

@@ -650,13 +650,14 @@ static void sbp2_login(struct work_struct *work)
 	if (sbp2_send_management_orb(lu, node_id, generation,
 				SBP2_LOGIN_REQUEST, lu->lun, &response) < 0) {
 		if (lu->retries++ < 5) {
-			queue_delayed_work(sbp2_wq, &lu->work,
-					   DIV_ROUND_UP(HZ, 5));
+			if (queue_delayed_work(sbp2_wq, &lu->work,
+					       DIV_ROUND_UP(HZ, 5)))
+				kref_get(&lu->tgt->kref);
 		} else {
 			fw_error("failed to login to %s LUN %04x\n",
 				 unit->device.bus_id, lu->lun);
-			kref_put(&lu->tgt->kref, sbp2_release_target);
 		}
+		kref_put(&lu->tgt->kref, sbp2_release_target);
 		return;
 	}
 
@@ -914,7 +915,9 @@ static void sbp2_reconnect(struct work_struct *work)
 			lu->retries = 0;
 			PREPARE_DELAYED_WORK(&lu->work, sbp2_login);
 		}
-		queue_delayed_work(sbp2_wq, &lu->work, DIV_ROUND_UP(HZ, 5));
+		if (queue_delayed_work(sbp2_wq, &lu->work, DIV_ROUND_UP(HZ, 5)))
+			kref_get(&lu->tgt->kref);
+		kref_put(&lu->tgt->kref, sbp2_release_target);
 		return;
 	}