[Ohai 6] Fix lsblk calls

lsblk doesn't qualify device names, sadly
Maybe...

```ruby
%w{/dev /dev/mapper}.each do |dir|
  path = File.join(dir, name)
  return path if File.exist?(path)
end
```

That's a bit more idiomatic ruby, avoids string interpolation, uses File.join and switches out deprecated exists? for exist?