block, bfq: fix possible UAF for bfqq->bic with merge chain

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

block, bfq: fix possible UAF for bfqq->bic with merge chain

1) initial state, three tasks:

Process 1 Process 2 Process 3
(BIC1) (BIC2) (BIC3)
| Λ | Λ | Λ
| | | | | |
V | V | V |
bfqq1 bfqq2 bfqq3
process ref: 1 1 1

2) bfqq1 merged to bfqq2:

Process 1 Process 2 Process 3
(BIC1) (BIC2) (BIC3)
| | | Λ
\--------------\| | |
V V |
bfqq1--------->bfqq2 bfqq3
process ref: 0 2 1

3) bfqq2 merged to bfqq3:

Process 1 Process 2 Process 3
(BIC1) (BIC2) (BIC3)
here -> Λ | |
\--------------\ \-------------\|
V V
bfqq1--------->bfqq2---------->bfqq3
process ref: 0 1 3

In this case, IO from Process 1 will get bfqq2 from BIC1 first, and then
get bfqq3 through merge chain, and finially handle IO by bfqq3.
Howerver, current code will think bfqq2 is owned by BIC1, like initial
state, and set bfqq2->bic to BIC1.

bfq_insert_request
-> by Process 1
bfqq = bfq_init_rq(rq)
bfqq = bfq_get_bfqq_handle_split
bfqq = bic_to_bfqq
-> get bfqq2 from BIC1
bfqq->ref++
rq->elv.priv[0] = bic
rq->elv.priv[1] = bfqq
if (bfqq_process_refs(bfqq) == 1)
bfqq->bic = bic
-> record BIC1 to bfqq2

__bfq_insert_request
new_bfqq = bfq_setup_cooperator
-> get bfqq3 from bfqq2->new_bfqq
bfqq_request_freed(bfqq)
new_bfqq->ref++
rq->elv.priv[1] = new_bfqq
-> handle IO by bfqq3

Fix the problem by checking bfqq is from merge chain fist. And this
might fix a following problem reported by our syzkaller(unreproducible):

==================================================================
BUG: KASAN: slab-use-after-free in bfq_do_early_stable_merge block/bfq-iosched.c:5692 [inline]
BUG: KASAN: slab-use-after-free in bfq_do_or_sched_stable_merge block/bfq-iosched.c:5805 [inline]
BUG: KASAN: slab-use-after-free in bfq_get_queue+0x25b0/0x2610 block/bfq-iosched.c:5889
Write of size 1 at addr ffff888123839eb8 by task kworker/0:1H/18595

CPU: 0 PID: 18595 Comm: kworker/0:1H Tainted: G L 6.6.0-07439-gba2303cacfda #6
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
Workqueue: kblockd blk_mq_requeue_work
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x91/0xf0 lib/dump_stack.c:106
print_address_description mm/kasan/report.c:364 [inline]
print_report+0x10d/0x610 mm/kasan/report.c:475
kasan_report+0x8e/0xc0 mm/kasan/report.c:588
bfq_do_early_stable_merge block/bfq-iosched.c:5692 [inline]
bfq_do_or_sched_stable_merge block/bfq-iosched.c:5805 [inline]
bfq_get_queue+0x25b0/0x2610 block/bfq-iosched.c:5889
bfq_get_bfqq_handle_split+0x169/0x5d0 block/bfq-iosched.c:6757
bfq_init_rq block/bfq-iosched.c:6876 [inline]
bfq_insert_request block/bfq-iosched.c:6254 [inline]
bfq_insert_requests+0x1112/0x5cf0 block/bfq-iosched.c:6304
blk_mq_insert_request+0x290/0x8d0 block/blk-mq.c:2593
blk_mq_requeue_work+0x6bc/0xa70 block/blk-mq.c:1502
process_one_work kernel/workqueue.c:2627 [inline]
process_scheduled_works+0x432/0x13f0 kernel/workqueue.c:2700
worker_thread+0x6f2/0x1160 kernel/workqueue.c:2781
kthread+0x33c/0x440 kernel/kthread.c:388
ret_from_fork+0x4d/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1b/0x30 arch/x86/entry/entry_64.S:305
</TASK>

Allocated by task 20776:
kasan_save_stack+0x20/0x40 mm/kasan/common.c:45
kasan_set_track+0x25/0x30 mm/kasan/common.c:52
__kasan_slab_alloc+0x87/0x90 mm/kasan/common.c:328
kasan_slab_alloc include/linux/kasan.h:188 [inline]
slab_post_alloc_hook mm/slab.h:763 [inline]
slab_alloc_node mm/slub.c:3458 [inline]
kmem_cache_alloc_node+0x1a4/0x6f0 mm/slub.c:3503
ioc_create_icq block/blk-ioc.c:370 [inline]
---truncated---