Bluetooth: hci_sock: fix slab oob read in create_monitor_event

When accessing hdev->name, the actual string length should prevail

Reported-by:  <syzbot+c90849c50ed209d77689@syzkaller.appspotmail.com>
Fixes: dcda1657

 ("Bluetooth: hci_core: Fix build warnings")
Signed-off-by: Edward AD <twuufnxlz@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>