Bluetooth: bnep: pin L2CAP connection during netdev registration

Bluetooth: bnep: pin L2CAP connection during netdev registration

bnep_add_connection() reads the L2CAP connection without holding the
channel lock, then passes its HCI device to register_netdev(). Controller
teardown can clear and release that connection concurrently, leaving the
network device registration path to dereference a freed parent device.

Take a reference to the L2CAP connection while holding the channel lock.
Retain it until register_netdev() has taken the parent device reference.

Fixes: 65f53e9802db ("Bluetooth: Access BNEP session addresses through L2CAP channel")
Reported-by: syzbot+fed5dce4553262f3b35c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fed5dce4553262f3b35c
Cc: stable@vger.kernel.org
Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>