sched: don't use skb queue helpers

A followup change will replace the sk_buff_head in the qdisc
struct with a slightly different list.

Use of the sk_buff_head helpers will thus cause compiler
warnings.

Open-code these accesses in an extra change to ease review.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>