rbd: cancel lock_dwork if the wait is interrupted

There is a warning message in my test with below steps:

  # rbd bench --io-type write --io-size 4K --io-threads 1 --io-pattern rand test &
  # sleep 5
  # pkill -9 rbd
  # rbd map test &
  # sleep 5
  # pkill rbd

The reason is that the rbd_add_acquire_lock() is interruptable,
that means, when we kill the waiting on ->acquire_wait, the lock_dwork
could be still running.

1. do_rbd_add()					2. lock_dwork
rbd_add_acquire_lock()
  - queue_delayed_work()
						lock_dwork queued
    - wait_for_completion_killable_timeout()  <-- kill happen
rbd_dev_image_unlock()	<-- UNLOCKED now, nothing to do.
rbd_dev_device_release()
rbd_dev_image_release()
  - ...
						lock successed here
     - cancel_delayed_work_sync(&rbd_dev->lock_dwork)

Then when we reach the rbd_dev_free(), WARN_ON is triggered because
lock_state is not RBD_LOCK_STATE_UNLOCKED.

To fix it, this commit make sure the lock_dwork was finished before
calling rbd_dev_image_unlock().

On the other hand, this would not happend in do_rbd_remove(), because
after rbd mapped, lock_dwork will only be queued for IO request, and
request will continue unless lock_dwork finished. when we call
rbd_dev_image_unlock() in do_rbd_remove(), all requests are done.
That means, lock_state should not be locked again after
rbd_dev_image_unlock().

[ Cancel lock_dwork in rbd_add_acquire_lock(), only if the wait is
  interrupted. ]

Fixes: 637cd060 ("rbd: new exclusive lock wait/wake code")
Signed-off-by: Dongsheng Yang <dongsheng.yang@easystack.cn>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>