tipc: fix problem with parallel link synchronization mechanism

Currently, we try to accumulate arrived packets in the links's
'deferred' queue during the parallel link syncronization phase.

This entails two problems:

- With an unlucky combination of arriving packets the algorithm
may go into a lockstep with the out-of-sequence handling function,
where the synch mechanism is adding a packet to the deferred queue,
while the out-of-sequence handling is retrieving it again, thus
ending up in a loop inside the node_lock scope.

- Even if this is avoided, the link will very often send out
unnecessary protocol messages, in the worst case leading to
redundant retransmissions.

We fix this by just dropping arriving packets on the upcoming link
during the synchronization phase, thus relying on the retransmission
protocol to resolve the situation once the two links have arrived to
a synchronized state.

Reviewed-by: Erik Hugne <[e-mail]>
Reviewed-by: Ying Xue <[e-mail]>
Signed-off-by: Jon Maloy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/tipc/link.c

···
 		}
 		/* Synchronize with parallel link if applicable */
 		if (unlikely((l_ptr->flags & LINK_SYNCHING) && !msg_dup(msg))) {
-			link_handle_out_of_seq_msg(l_ptr, skb);
-			if (link_synch(l_ptr))
-				link_retrieve_defq(l_ptr, &head);
-			skb = NULL;
-			goto unlock;
+			if (!link_synch(l_ptr))
+				goto unlock;
 		}
 		l_ptr->next_in_no++;
 		if (unlikely(!skb_queue_empty(&l_ptr->deferdq)))