Bluetooth: fix use-after-free in device_for_each_child()

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

Bluetooth: fix use-after-free in device_for_each_child()

Syzbot has reported the following KASAN splat:

BUG: KASAN: slab-use-after-free in device_for_each_child+0x18f/0x1a0
Read of size 8 at addr ffff88801f605308 by task kbnepd bnep0/4980

CPU: 0 UID: 0 PID: 4980 Comm: kbnepd bnep0 Not tainted 6.12.0-rc4-00161-gae90f6a6170d #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-2.fc40 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x100/0x190
? device_for_each_child+0x18f/0x1a0
print_report+0x13a/0x4cb
? __virt_addr_valid+0x5e/0x590
? __phys_addr+0xc6/0x150
? device_for_each_child+0x18f/0x1a0
kasan_report+0xda/0x110
? device_for_each_child+0x18f/0x1a0
? __pfx_dev_memalloc_noio+0x10/0x10
device_for_each_child+0x18f/0x1a0
? __pfx_device_for_each_child+0x10/0x10
pm_runtime_set_memalloc_noio+0xf2/0x180
netdev_unregister_kobject+0x1ed/0x270
unregister_netdevice_many_notify+0x123c/0x1d80
? __mutex_trylock_common+0xde/0x250
? __pfx_unregister_netdevice_many_notify+0x10/0x10
? trace_contention_end+0xe6/0x140
? __mutex_lock+0x4e7/0x8f0
? __pfx_lock_acquire.part.0+0x10/0x10
? rcu_is_watching+0x12/0xc0
? unregister_netdev+0x12/0x30
unregister_netdevice_queue+0x30d/0x3f0
? __pfx_unregister_netdevice_queue+0x10/0x10
? __pfx_down_write+0x10/0x10
unregister_netdev+0x1c/0x30
bnep_session+0x1fb3/0x2ab0
? __pfx_bnep_session+0x10/0x10
? __pfx_lock_release+0x10/0x10
? __pfx_woken_wake_function+0x10/0x10
? __kthread_parkme+0x132/0x200
? __pfx_bnep_session+0x10/0x10
? kthread+0x13a/0x370
? __pfx_bnep_session+0x10/0x10
kthread+0x2b7/0x370
? __pfx_kthread+0x10/0x10
ret_from_fork+0x48/0x80
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1a/0x30
</TASK>

Allocated by task 4974:
kasan_save_stack+0x30/0x50
kasan_save_track+0x14/0x30
__kasan_kmalloc+0xaa/0xb0
__kmalloc_noprof+0x1d1/0x440
hci_alloc_dev_priv+0x1d/0x2820
__vhci_create_device+0xef/0x7d0
vhci_write+0x2c7/0x480
vfs_write+0x6a0/0xfc0
ksys_write+0x12f/0x260
do_syscall_64+0xc7/0x250
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 4979:
kasan_save_stack+0x30/0x50
kasan_save_track+0x14/0x30
kasan_save_free_info+0x3b/0x60
__kasan_slab_free+0x4f/0x70
kfree+0x141/0x490
hci_release_dev+0x4d9/0x600
bt_host_release+0x6a/0xb0
device_release+0xa4/0x240
kobject_put+0x1ec/0x5a0
put_device+0x1f/0x30
vhci_release+0x81/0xf0
__fput+0x3f6/0xb30
task_work_run+0x151/0x250
do_exit+0xa79/0x2c30
do_group_exit+0xd5/0x2a0
get_signal+0x1fcd/0x2210
arch_do_signal_or_restart+0x93/0x780
syscall_exit_to_user_mode+0x140/0x290
do_syscall_64+0xd4/0x250
entry_SYSCALL_64_after_hwframe+0x77/0x7f

In 'hci_conn_del_sysfs()', 'device_unregister()' may be called when
an underlying (kobject) reference counter is greater than 1. This
means that reparenting (happened when the device is actually freed)
is delayed and, during that delay, parent controller device (hciX)
may be deleted. Since the latter may create a dangling pointer to
freed parent, avoid that scenario by reparenting to NULL explicitly.