sch_htb: fix doc warning in htb_next_rb_node()

Add description for parameters of htb_next_rb_node() to fix
gcc W=1 warnings:

net/sched/sch_htb.c:339: warning: Function parameter or member 'n' not described in 'htb_next_rb_node'
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_htb.c

@@ -337,6 +337,7 @@ static void htb_add_to_wait_tree(struct htb_sched *q,
 
 /**
  * htb_next_rb_node - finds next node in binary tree
+ * @n: the current node in binary tree
  *
  * When we are past last key we return NULL.
  * Average complexity is 2 steps per call.