- Replace per-namespace recursive lock with an ldlm-global lock, to close the

- Replace per-namespace recursive lock with an ldlm-global lock, to close the
  race between ldlm_handle2lock and the lock destroy portion of ldlm_lock_put
- Add a special interface for ldlm_handle2lock to resolve the following
  conflict between our invariants:

  1. ldlm_handle2lock should never give a reference to a "dying" lock
  2. ldlm_lock_decref needs to use ldlm_handle2lock to get a reference to a lock
     so that it can decrease the readers/writers refcount
  3. The lock can't finish dying until readers/writers hits zero.

  That should be fixed now.