loop-util: lock the control device around clearing the loopback device and deleting it

loop-util: lock the control device around clearing the loopback device and deleting it

This mirrors what we already do during allocation. We lock the control
device first, and then release the block device and then delete it.

This makes things substantially more robust as long all participants do
such locking: we won't attempt to delete a block device somebody else
already is using.