fail when probing root fs with btrfs

When the root fs is a btrfs volume c-b-m will fail to probe it.

```cbm_probe_path()``` will call ```cbm_devnode_to_devpath()```, there we assert the major number and assume it should be greater than 0, but in the case of btrfs the major will be always 0 (due its pseudo-fs nature). We must implement an alternate case when we realize we're dealing with btrfs.