Bluetooth: Fix a refcnt underflow problem for hci_conn

Syzbot reports a warning as follows:

WARNING: CPU: 1 PID: 26946 at net/bluetooth/hci_conn.c:619
hci_conn_timeout+0x122/0x210 net/bluetooth/hci_conn.c:619
...
Call Trace:
 <TASK>
 process_one_work+0x884/0x15c0 kernel/workqueue.c:2630
 process_scheduled_works kernel/workqueue.c:2703 [inline]
 worker_thread+0x8b9/0x1290 kernel/workqueue.c:2784
 kthread+0x33c/0x440 kernel/kthread.c:388
 ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:304
 </TASK>

It is because the HCI_EV_SIMPLE_PAIR_COMPLETE event handler drops
hci_conn directly without check Simple Pairing whether be enabled. But
the Simple Pairing process can only be used if both sides have the
support enabled in the host stack.

Add hci_conn_ssp_enabled() for hci_conn in HCI_EV_IO_CAPA_REQUEST and
HCI_EV_SIMPLE_PAIR_COMPLETE event handlers to fix the problem.

Fixes: 0493684e

 ("[Bluetooth] Disable disconnect timer during Simple Pairing")
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>