loop-util: when clearing a loopback device delete partitions first, and take BSD...

loop-util: when clearing a loopback device delete partitions first, and take BSD lock

Whenever we release a loopback device, let's first synchronously delete
all partitions, so that we know that's complete and not done
asynchronously in the background. Take a BSD lock on the device while
doing so, so that udev won't make the devices busy while we do this.