net/sched: accept TCA_STAB only for root qdisc

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

net/sched: accept TCA_STAB only for root qdisc

Most qdiscs maintain their backlog using qdisc_pkt_len(skb)
on the assumption it is invariant between the enqueue()
and dequeue() handlers.

Unfortunately syzbot can crash a host rather easily using
a TBF + SFQ combination, with an STAB on SFQ [1]

We can't support TCA_STAB on arbitrary level, this would
require to maintain per-qdisc storage.

[1]
[ 88.796496] BUG: kernel NULL pointer dereference, address: 0000000000000000
[ 88.798611] #PF: supervisor read access in kernel mode
[ 88.799014] #PF: error_code(0x0000) - not-present page
[ 88.799506] PGD 0 P4D 0
[ 88.799829] Oops: Oops: 0000 [#1] SMP NOPTI
[ 88.800569] CPU: 14 UID: 0 PID: 2053 Comm: b371744477 Not tainted 6.12.0-rc1-virtme #1117
[ 88.801107] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[ 88.801779] RIP: 0010:sfq_dequeue (net/sched/sch_sfq.c:272 net/sched/sch_sfq.c:499) sch_sfq
[ 88.802544] Code: 0f b7 50 12 48 8d 04 d5 00 00 00 00 48 89 d6 48 29 d0 48 8b 91 c0 01 00 00 48 c1 e0 03 48 01 c2 66 83 7a 1a 00 7e c0 48 8b 3a <4c> 8b 07 4c 89 02 49 89 50 08 48 c7 47 08 00 00 00 00 48 c7 07 00
All code
========
0: 0f b7 50 12 movzwl 0x12(%rax),%edx
4: 48 8d 04 d5 00 00 00 lea 0x0(,%rdx,8),%rax
b: 00
c: 48 89 d6 mov %rdx,%rsi
f: 48 29 d0 sub %rdx,%rax
12: 48 8b 91 c0 01 00 00 mov 0x1c0(%rcx),%rdx
19: 48 c1 e0 03 shl $0x3,%rax
1d: 48 01 c2 add %rax,%rdx
20: 66 83 7a 1a 00 cmpw $0x0,0x1a(%rdx)
25: 7e c0 jle 0xffffffffffffffe7
27: 48 8b 3a mov (%rdx),%rdi
2a:* 4c 8b 07 mov (%rdi),%r8 <-- trapping instruction
2d: 4c 89 02 mov %r8,(%rdx)
30: 49 89 50 08 mov %rdx,0x8(%r8)
34: 48 c7 47 08 00 00 00 movq $0x0,0x8(%rdi)
3b: 00
3c: 48 rex.W
3d: c7 .byte 0xc7
3e: 07 (bad)
...

Code starting with the faulting instruction
===========================================
0: 4c 8b 07 mov (%rdi),%r8
3: 4c 89 02 mov %r8,(%rdx)
6: 49 89 50 08 mov %rdx,0x8(%r8)
a: 48 c7 47 08 00 00 00 movq $0x0,0x8(%rdi)
11: 00
12: 48 rex.W
13: c7 .byte 0xc7
14: 07 (bad)
...
[ 88.803721] RSP: 0018:ffff9a1f892b7d58 EFLAGS: 00000206
[ 88.804032] RAX: 0000000000000000 RBX: ffff9a1f8420c800 RCX: ffff9a1f8420c800
[ 88.804560] RDX: ffff9a1f81bc1440 RSI: 0000000000000000 RDI: 0000000000000000
[ 88.805056] RBP: ffffffffc04bb0e0 R08: 0000000000000001 R09: 00000000ff7f9a1f
[ 88.805473] R10: 000000000001001b R11: 0000000000009a1f R12: 0000000000000140
[ 88.806194] R13: 0000000000000001 R14: ffff9a1f886df400 R15: ffff9a1f886df4ac
[ 88.806734] FS: 00007f445601a740(0000) GS:ffff9a2e7fd80000(0000) knlGS:0000000000000000
[ 88.807225] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 88.807672] CR2: 0000000000000000 CR3: 000000050cc46000 CR4: 00000000000006f0
[ 88.808165] Call Trace:
[ 88.808459] <TASK>
[ 88.808710] ? __die (arch/x86/kernel/dumpstack.c:421 arch/x86/kernel/dumpstack.c:434)
[ 88.809261] ? page_fault_oops (arch/x86/mm/fault.c:715)
[ 88.809561] ? exc_page_fault (./arch/x86/include/asm/irqflags.h:26 ./arch/x86/include/asm/irqflags.h:87 ./arch/x86/include/asm/irqflags.h:147 arch/x86/mm/fault.c:1489 arch/x86/mm/fault.c:1539)
[ 88.809806] ? asm_exc_page_fault (./arch/x86/include/asm/idtentry.h:623)
[ 88.810074] ? sfq_dequeue (net/sched/sch_sfq.c:272 net/sched/sch_sfq.c:499) sch_sfq
[ 88.810411] sfq_reset (net/sched/sch_sfq.c:525) sch_sfq
[ 88.810671] qdisc_reset (./include/linux/skbuff.h:2135 ./include/linux/skbuff.h:2441 ./include/linux/skbuff.h:3304 ./include/linux/skbuff.h:3310 net/sched/sch_g
---truncated---