sch_sfq: fix null pointer dereference at timer expiration

While converting sch_sfq to use timer_setup(), the commit cdeabbb
("net: sched: Convert timers to use timer_setup()") forgot to
initialize the 'sch' field. As a result, the timer callback tries to
dereference a NULL pointer, and the kernel does oops.

Fix it initializing such field at qdisc creation time.

Fixes: cdeabbb ("net: sched: Convert timers to use timer_setup()")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_sfq.c

@@ -724,6 +724,7 @@ static int sfq_init(struct Qdisc *sch, struct nlattr *opt)
 	int i;
 	int err;
 
+	q->sch = sch;
 	timer_setup(&q->perturb_timer, sfq_perturbation, TIMER_DEFERRABLE);
 
 	err = tcf_block_get(&q->block, &q->filter_list, sch);