devmapper: Fail device deletion early if device is still mounted
If a device is still mounted at the time of DeleteDevice(), that means
higher layers have not called Put() properly on the device and are trying
to delete it. This is a bug in the code where Get() and Put() have not been
properly paired up. Fail device deletion if it is still mounted.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>