The recent slab allocation changes mean that we no longer keep a
permanent list of commands on the device_queue list.  However,
certain pieces of SCSI code relied on being able to traverse this
list to find details of all outstanding commands (the error handler
being the prime example).  This code adds back a new dynamic cmd_list
which keeps the list of commands currently allocated to the device.
Since the list is dynamic, it is protected by a lock (list_lock).