Fix $service_name in Debian distributions to use 'smartmontools'

The service has been called `smartmontools` in Debian for a while but the module by default calls it `smartd`. 

See `/etc/init.d/smartmontools` in:
https://packages.debian.org/wheezy/amd64/smartmontools/filelist
https://packages.debian.org/sid/amd64/smartmontools/filelist
http://packages.ubuntu.com/lucid/amd64/smartmontools/filelist
http://packages.ubuntu.com/trusty/amd64/smartmontools/filelist