Bluetooth: hci_sysfs: Fix NULL pointer dereference in device_del()

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

Bluetooth: hci_sysfs: Fix NULL pointer dereference in device_del()

A NULL pointer dereference in klist_put() occurs when a child device (such
as a BNEP network device in bnep_session) is concurrently being
unregistered while hci_conn_del_sysfs() reparents child devices.

This is caused by a race condition between hci_conn_del_sysfs() and
concurrent child device unregistration (e.g. bnep_session calling
unregister_netdev()). During device unregistration, device_del() snapshots
a non-NULL parent pointer. Concurrently, hci_conn_del_sysfs() finds the
child device using device_find_any_child() and calls device_move() to
reparent it to NULL, which removes the node from its parent's klist and
clears knode_parent. Subsequently, device_del() calls
klist_del(&dev->p->knode_parent) using the stale parent snapshot, causing
klist_put() to dereference knode_klist(n)->put on an already removed node,
resulting in a NULL pointer dereference.

This race was introduced by commit 27aabf27fd01 ("Bluetooth: fix
use-after-free in device_for_each_child()"), which replaced
device_find_child(..., __match_tty) with device_find_any_child() in
hci_conn_del_sysfs(). That change was intended to avoid a use-after-free
where conn->dev outlived its parent hdev->dev when child devices held
references to conn->dev, because conn->dev only held a reference to
hdev->dev while registered in sysfs.

Fix the issue properly by taking an explicit reference to the parent device
with get_device(&hdev->dev) in hci_conn_init_sysfs() and dropping it with
put_device(parent) in bt_link_release() when the conn device is freed. This
ensures that hdev->dev remains valid for the entire lifecycle of conn->dev,
resolving the underlying use-after-free. With the parent reference held
properly, restore the __match_tty filter in hci_conn_del_sysfs() so that
device_move() is only invoked on persistent RFCOMM TTY devices as
originally intended, eliminating the race condition with unregistering
network devices.