net: sched: cake: fix null pointer access issue when cake_init() fails

When the default qdisc is cake, if the qdisc of dev_queue fails to be
inited during mqprio_init(), cake_reset() is invoked to clear
resources. In this case, the tins is NULL, and it will cause gpf issue.

The process is as follows:
qdisc_create_dflt()
	cake_init()
		q->tins = kvcalloc(...)        --->failed, q->tins is NULL
	...
	qdisc_put()
		...
		cake_reset()
			...
			cake_dequeue_one()
				b = &q->tins[...]   --->q->tins is NULL

The following is the Call Trace information:
general protection fault, probably for non-canonical address
0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
RIP: 0010:cake_dequeue_one+0xc9/0x3c0
Call Trace:
<TASK>
cake_reset+0xb1/0x140
qdisc_reset+0xed/0x6f0
qdisc_destroy+0x82/0x4c0
qdisc_put+0x9e/0xb0
qdisc_create_dflt+0x2c3/0x4a0
mqprio_init+0xa71/0x1760
qdisc_create+0x3eb/0x1000
tc_modify_qdisc+0x408/0x1720
rtnetlink_rcv_msg+0x38e/0xac0
netlink_rcv_skb+0x12d/0x3a0
netlink_unicast+0x4a2/0x740
netlink_sendmsg+0x826/0xcc0
sock_sendmsg+0xc5/0x100
____sys_sendmsg+0x583/0x690
___sys_sendmsg+0xe8/0x160
__sys_sendmsg+0xbf/0x160
do_syscall_64+0x35/0x80
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f89e5122d04
</TASK>

Fixes: 046f6fd5 ("sched: Add Common Applications Kept Enhanced (cake) qdisc")
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_cake.c

@@ -2224,8 +2224,12 @@ static struct sk_buff *cake_dequeue(struct Qdisc *sch)
 
 static void cake_reset(struct Qdisc *sch)
 {
+	struct cake_sched_data *q = qdisc_priv(sch);
 	u32 c;
 
+	if (!q->tins)
+		return;
+
 	for (c = 0; c < CAKE_MAX_TINS; c++)
 		cake_clear_tin(sch, c);
 }