Bluetooth: btmtk: avoid UAF in btmtk_process_coredump

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

Bluetooth: btmtk: avoid UAF in btmtk_process_coredump

hci_devcd_append may lead to the release of the skb, so it cannot be
accessed once it is called.

==================================================================
BUG: KASAN: slab-use-after-free in btmtk_process_coredump+0x2a7/0x2d0 [btmtk]
Read of size 4 at addr ffff888033cfabb0 by task kworker/0:3/82

CPU: 0 PID: 82 Comm: kworker/0:3 Tainted: G U 6.6.40-lockdep-03464-g1d8b4eb3060e #1 b0b3c1cc0c842735643fb411799d97921d1f688c
Hardware name: Google Yaviks_Ufs/Yaviks_Ufs, BIOS Google_Yaviks_Ufs.15217.552.0 05/07/2024
Workqueue: events btusb_rx_work [btusb]
Call Trace:
<TASK>
dump_stack_lvl+0xfd/0x150
print_report+0x131/0x780
kasan_report+0x177/0x1c0
btmtk_process_coredump+0x2a7/0x2d0 [btmtk 03edd567dd71a65958807c95a65db31d433e1d01]
btusb_recv_acl_mtk+0x11c/0x1a0 [btusb 675430d1e87c4f24d0c1f80efe600757a0f32bec]
btusb_rx_work+0x9e/0xe0 [btusb 675430d1e87c4f24d0c1f80efe600757a0f32bec]
worker_thread+0xe44/0x2cc0
kthread+0x2ff/0x3a0
ret_from_fork+0x51/0x80
ret_from_fork_asm+0x1b/0x30
</TASK>

Allocated by task 82:
stack_trace_save+0xdc/0x190
kasan_set_track+0x4e/0x80
__kasan_slab_alloc+0x4e/0x60
kmem_cache_alloc+0x19f/0x360
skb_clone+0x132/0xf70
btusb_recv_acl_mtk+0x104/0x1a0 [btusb]
btusb_rx_work+0x9e/0xe0 [btusb]
worker_thread+0xe44/0x2cc0
kthread+0x2ff/0x3a0
ret_from_fork+0x51/0x80
ret_from_fork_asm+0x1b/0x30

Freed by task 1733:
stack_trace_save+0xdc/0x190
kasan_set_track+0x4e/0x80
kasan_save_free_info+0x28/0xb0
____kasan_slab_free+0xfd/0x170
kmem_cache_free+0x183/0x3f0
hci_devcd_rx+0x91a/0x2060 [bluetooth]
worker_thread+0xe44/0x2cc0
kthread+0x2ff/0x3a0
ret_from_fork+0x51/0x80
ret_from_fork_asm+0x1b/0x30

The buggy address belongs to the object at ffff888033cfab40
which belongs to the cache skbuff_head_cache of size 232
The buggy address is located 112 bytes inside of
freed 232-byte region [ffff888033cfab40, ffff888033cfac28)

The buggy address belongs to the physical page:
page:00000000a174ba93 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x33cfa
head:00000000a174ba93 order:1 entire_mapcount:0 nr_pages_mapped:0 pincount:0
anon flags: 0x4000000000000840(slab|head|zone=1)
page_type: 0xffffffff()
raw: 4000000000000840 ffff888100848a00 0000000000000000 0000000000000001
raw: 0000000000000000 0000000080190019 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff888033cfaa80: fb fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc
ffff888033cfab00: fc fc fc fc fc fc fc fc fa fb fb fb fb fb fb fb
>ffff888033cfab80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff888033cfac00: fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc
ffff888033cfac80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Check if we need to call hci_devcd_complete before calling
hci_devcd_append. That requires that we check data->cd_info.cnt >=
MTK_COREDUMP_NUM instead of data->cd_info.cnt > MTK_COREDUMP_NUM, as we
increment data->cd_info.cnt only once the call to hci_devcd_append
succeeds.