Bluetooth: mgmt: hold reference for hci_conn in mgmt_pending_cmds

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: mgmt: hold reference for hci_conn in mgmt_pending_cmds Dereferencing RCU-protected pointers outside critical sections is invalid and may lead to UAF. Use of hci_conn in hci_sync callbacks also needs to hold refcount to avoid UAF. Take appropriate locks for hci_conn lookups, and take refcount for hci_conn pointers stored in mgmt_pending_cmd so that the pointer stays valid. When accessing conn->state, ensure hdev->lock is held to avoid data race.