bluetooth: hci_sysfs: Fix attempting to call device_add multiple times

device_add shall not be called multiple times as stated in its
documentation:

 'Do not call this routine or device_register() more than once for
 any device structure'

Syzkaller reports a bug as follows [1]:
------------[ cut here ]------------
kernel BUG at lib/list_debug.c:33!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
[...]
Call Trace:
 <TASK>
 __list_add include/linux/list.h:69 [inline]
 list_add_tail include/linux/list.h:102 [inline]
 kobj_kset_join lib/kobject.c:164 [inline]
 kobject_add_internal+0x18f/0x8f0 lib/kobject.c:214
 kobject_add_varg lib/kobject.c:358 [inline]
 kobject_add+0x150/0x1c0 lib/kobject.c:410
 device_add+0x368/0x1e90 drivers/base/core.c:3452
 hci_conn_add_sysfs+0x9b/0x1b0 net/bluetooth/hci_sysfs.c:53
 hci_le_cis_estabilished_evt+0x57c/0xae0 net/bluetooth/hci_event.c:6799
 hci_le_meta_evt+0x2b8/0x510 net/bluetooth/hci_event.c:7110
 hci_event_func net/bluetooth/hci_event.c:7440 [inline]
 hci_event_packet+0x63d/0xfd0 net/bluetooth/hci_event.c:7495
 hci_rx_work+0xae7/0x1230 net/bluetooth/hci_core.c:4007
 process_one_work+0x991/0x1610 kernel/workqueue.c:2289
 worker_thread+0x665/0x1080 kernel/workqueue.c:2436
 kthread+0x2e4/0x3a0 kernel/kthread.c:376
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:306
 </TASK>

Link: https://syzkaller.appspot.com/bug?id=da3246e2d33afdb92d66bc166a0934c5b146404a
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Tested-by: Hawkins Jiawei <yin31149@gmail.com>
---
diff -ruN linux-5.14.0-427.18.orig/net/bluetooth/hci_sysfs.c linux-5.14.0-427.18/net/bluetooth/hci_sysfs.c
--- linux-5.14.0-427.18.orig/net/bluetooth/hci_sysfs.c
+++ linux-5.14.0-427.18/net/bluetooth/hci_sysfs.c
@@ -48,6 +48,9 @@ void hci_conn_add_sysfs(struct hci_conn *conn)
 
 	bt_dev_dbg(hdev, "conn %p", conn);
 
+	if (device_is_registered(&conn->dev))
+		return;
+
 	if (device_is_registered(&conn->dev))
 		return;