Posix locks don't work between threads, but it seems some programs do
    involve threads sharing plocks incidentally (with meaningless results).
    Given the plock state we keep in lock_dlm, multiple threads accessing the
    locks would not only be meaningless, but could corrupt the state, leaving
    the threads permanently hung.  This patch tries to keep threads from
    stomping on each other, hopefully preventing most hangs.
    
    bz 383391