uorb: fix several race conditions during topic initialization

Possible race conditions (they all happen between the check of existence
of a topic and trying to create the node):
- single instance, with multiple advertisers during the first advertise:
  both advertisers see the topic as non-existent and try to advertise it.
  One of them will fail, leading to an error message.
  This is the cause for telemetry_status advert failure seen in SITL in
  rare cases.
- multi-instance: subscription to non-existing instance -> px4_open fails,
  and the subscriber tries to create the node. If during that time a
  publisher publishes that instance, the subscriber will get (instance+1)
  (or fails if the max number of instances is exceeded).
  This is a race that goes pretty much unnoticed.
- multi-instance: 2 publishers can get the same instance (if is_published()
  is false in case both have not published data yet).
  This can also go unnoticed.
  Therefore the patch changes where _advertised is set: it is now set
  directly during the advertisement instead of during publication.

src/modules/uORB/uORBDeviceNode.cpp

@@ -90,6 +90,7 @@ uORB::DeviceNode::open(cdev::file_t *filp)
 			ret = -EBUSY;
 		}
 
+		mark_as_advertised();
 		unlock();
 
 		/* now complete the open */
@@ -306,7 +307,6 @@ uORB::DeviceNode::write(cdev::file_t *filp, const char *buffer, size_t buflen)
 	/* update the timestamp and generation count */
 	_last_update = hrt_absolute_time();
 
-	_advertised = true;
 
 	// callbacks
 	for (auto item : _callbacks) {