filesystem2 bugfix
filesystem2 bugfix

This makes 'mounts' in by_device and 'devices' in by_mount always get
initialized as an empty array instead of not existing. if the data doesn't
exist. This prevents callers from having to do

```
if foo and foo['mounts'] and foo['mounts'].emtpy?
```

to determine if something is mounted.

n-marton/ohai,mattray/ohai,Juniper/ohai,dreamnite/ohai,numericillustration/ohai,chef/ohai,acaiafa/ohai,shtzr840329/ohai,tas50/ohai-1,johnbellone/ohai,shtzr840329/ohai,johnbellone/ohai,juliandunn/ohai,Fodoj/ohai,higanworks/ohai,jseely/ohai,sh9189/ohai,numericillustration/ohai,375gnu/ohai,zshuo/ohai,zshuo/ohai,sh9189/ohai,acaiafa/ohai,bougyman/ohai,jaymzh/ohai,davide125/ohai,Fodoj/ohai,glennmatthews/ohai,higanworks/ohai,juliandunn/ohai,mattray/ohai,tas50/ohai-1,jaymzh/ohai,phreakocious/ohai,bougyman/ohai,adamleff/ohai,davide125/ohai,n-marton/ohai,Juniper/ohai,dreamnite/ohai,phreakocious/ohai,jseely/ohai,375gnu/ohai,http-418/ohai,glennmatthews/ohai,adamleff/ohai,http-418/ohai,chef/ohai,criteo-forks/ohai,criteo-forks/ohai