tipc: fix crash during node removal

When the TIPC module is unloaded, we have identified a race condition
that allows a node reference counter to go to zero and the node instance
being freed before the node timer is finished with accessing it. This
leads to occasional crashes, especially in multi-namespace environments.

The scenario goes as follows:

CPU0:(node_stop)                       CPU1:(node_timeout)  // ref == 2

1:                                          if(!mod_timer())
2: if (del_timer())
3:   tipc_node_put()                                        // ref -> 1
4: tipc_node_put()                                          // ref -> 0
5:   kfree_rcu(node);
6:                                               tipc_node_get(node)
7:                                               // BOOM!

We now clean up this functionality as follows:

1) We remove the node pointer from the node lookup table before we
   attempt deactivating the timer. This way, we reduce the risk that
   tipc_node_find() may obtain a valid pointer to an instance marked
   for deletion; a harmless but undesirable situation.

2) We use del_timer_sync() instead of del_timer() to safely deactivate
   the node timer without any risk that it might be reactivated by the
   timeout handler. There is no risk of deadlock here, since the two
   functions never touch the same spinlocks.

3: We remove a pointless tipc_node_get() + tipc_node_put() from the
   timeout handler.

Reported-by: Zhijiang Hu <huzhijiang@gmail.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>