rbd: don't move requests to the running list on errors

The running list is supposed to contain requests that are pinning the
exclusive lock, i.e. those that must be flushed before exclusive lock
is released.  When wake_lock_waiters() is called to handle an error,
requests on the acquiring list are failed with that error and no
flushing takes place.  Briefly moving them to the running list is not
only pointless but also harmful: if exclusive lock gets acquired
before all of their state machines are scheduled and go through
rbd_lock_del_request(), we trigger

    rbd_assert(list_empty(&rbd_dev->running_list));

in rbd_try_acquire_lock().

Cc: stable@vger.kernel.org
Fixes: 637cd060

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