clusterer: fix possible inconsistent list handling

The tags `active_msgs_sent` list might become inconsistent due to the
fact that the `ni->next` is being populated outside locks.

modules/clusterer/sharing_tags.c

@@ -739,8 +739,8 @@ int shtag_activate(str *tag_name, int cluster_id, char *reason, int reason_len)
  return ret;
  }
  ni->node_id = node->node_id;
- ni->next = tag->active_msgs_sent;
  lock_switch_write(shtags_lock, lock_old_flag);
+ ni->next = tag->active_msgs_sent;
  tag->active_msgs_sent = ni;
  lock_switch_read(shtags_lock, lock_old_flag);
  }
@@ -816,8 +816,8 @@ void shtag_flush_state(int c_id, int node_id)
  return;
  }
  ni->node_id = node_id;
- ni->next = tag->active_msgs_sent;
  lock_switch_write(shtags_lock, lock_old_flag);
+ ni->next = tag->active_msgs_sent;
  tag->active_msgs_sent = ni;
  lock_switch_read(shtags_lock, lock_old_flag);
  }