blockdev-util: don't use mixed style of retval in block_device_get_originating

blockdev-util: don't use mixed style of retval in block_device_get_originating

We have two typical styles of 'ret' param assignment + retval:

1) < 0 on actual error, 0 on nothing (ret == NULL), > 0 on something
2) recognizable errno on nothing, < 0 on other errors, >= 0 on something

but never use both at the same time.