Thinking a bit more, I really don't think the existing any refcount
check protects us against a different tread modififying the backing
file.  When a process has a file descriptor to a loop device open
and is multithreaded (or forks) we can still have multiple threads
manipulating the loop state.

That being said I do not think we really need that refcount check
at all - once loop_clr_fd set lo->lo_state to Lo_rundown under the
global lock we know that loop_validate_file will error out on it
due to the lo_state != Lo_bound check.