We actually fix this race by mediating the object release/get race
(i.e. we destroy the scsi_disk object when its reference count goes
1->0, we use a semaphore to prevent something else trying to get a
reference after or during this).

The open/remove race is actually irrelevant because even if we open an
already removed object, all that will happen is that we get a
reference to a device that always returns EIO.