Add support for Linux SAS classes

[scottlaird]

Linux exports ~5 useful metrics per SAS PHY, which can be useful for debugging SAS performance problems and errors. I'm planning on adding support for this to node_exporter (see https://github.com/prometheus/node_exporter/issues/2386), but I need to extract information from /sys first before I can do this.

The useful metrics are all in /sys/class/sas_phy, but we need data from /sys/class/sas_end_device, /sys/class/sas_port, and /sys/class/sas_expander in order to make sense of the PHY data and know what's connected to a specific PHY.