losetup -d only if the loop device is still set up

Commit 2d0e9fcecd3e57215d42a5c128654c48de787aba introduced a workaround
for kpartx -d failing to correctly remove partition mappings.  However,
when kpartx -d functions correctly, losetup -d fails.

Now, calls losetup -d only if the backing_file map exists.

Signed-off-by: Michael K Johnson <johnsonm@foresightlinux.org>

Fixes #21