net/sched: sch_sfb: Replace direct dequeue call with peek and qdisc_dequeue_peeked

In the Linux kernel, the following vulnerability has been resolved:

net/sched: sch_sfb: Replace direct dequeue call with peek and qdisc_dequeue_peeked

When sfb has children (eg qfq qdisc) whose peek() callback is
qdisc_peek_dequeued(), we could get a kernel panic. When the parent of such
qdiscs (eg illustrated in patch #3 as tbf) wants to retrieve an skb from
its child (sfb in this case), it will do the following:
1a. do a peek() - and when sensing there's an skb the child can offer, then
- the child in this case(sfb) calls its child's (qfq) peek.
qfq does the right thing and will return the gso_skb queue packet.
Note: if there wasnt a gso_skb entry then qfq will store it there.
1b. invoke a dequeue() on the child (sfb). And herein lies the problem.
- sfb will call the child's dequeue() which will essentially just
try to grab something of qfq's queue.

[ 127.594489][ T453] KASAN: null-ptr-deref in range [0x0000000000000048-0x000000000000004f]
[ 127.594741][ T453] CPU: 2 UID: 0 PID: 453 Comm: ping Not tainted 7.1.0-rc1-00035-gac961974495b-dirty #793 PREEMPT(full)
[ 127.595059][ T453] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 127.595254][ T453] RIP: 0010:qfq_dequeue+0x35c/0x1650 [sch_qfq]
[ 127.595461][ T453] Code: 00 fc ff df 80 3c 02 00 0f 85 17 0e 00 00 4c 8d 73 48 48 89 9d b8 02 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <80> 3c 02 00 0f 85 76 0c 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b
[ 127.596081][ T453] RSP: 0018:ffff88810e5af440 EFLAGS: 00010216
[ 127.596337][ T453] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: dffffc0000000000
[ 127.596623][ T453] RDX: 0000000000000009 RSI: 0000001880000000 RDI: ffff888104fd82b0
[ 127.596917][ T453] RBP: ffff888104fd8000 R08: ffff888104fd8280 R09: 1ffff110211893a3
[ 127.597165][ T453] R10: 1ffff110211893a6 R11: 1ffff110211893a7 R12: 0000001880000000
[ 127.597404][ T453] R13: ffff888104fd82b8 R14: 0000000000000048 R15: 0000000040000000
[ 127.597644][ T453] FS: 00007fc380cbfc40(0000) GS:ffff88816f2a8000(0000) knlGS:0000000000000000
[ 127.597956][ T453] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 127.598160][ T453] CR2: 00005610aa9890a8 CR3: 000000010369e000 CR4: 0000000000750ef0
[ 127.598390][ T453] PKRU: 55555554
[ 127.598509][ T453] Call Trace:
[ 127.598629][ T453] <TASK>
[ 127.598718][ T453] ? mark_held_locks+0x40/0x70
[ 127.598890][ T453] ? srso_alias_return_thunk+0x5/0xfbef5
[ 127.599053][ T453] sfb_dequeue+0x88/0x4d0
[ 127.599174][ T453] ? ktime_get+0x137/0x230
[ 127.599328][ T453] ? srso_alias_return_thunk+0x5/0xfbef5
[ 127.599480][ T453] ? qdisc_peek_dequeued+0x7b/0x350 [sch_qfq]
[ 127.599670][ T453] ? srso_alias_return_thunk+0x5/0xfbef5
[ 127.599831][ T453] tbf_dequeue+0x6b1/0x1098 [sch_tbf]
[ 127.599988][ T453] __qdisc_run+0x169/0x1900

The right thing to do in #1b is to grab the skb off gso_skb queue.
This patchset fixes that issue by changing #1b to use qdisc_dequeue_peeked()
method instead.