nit: add missing `diskSelector` to `node_filesystem_files_free`

IIUC, this doesn't change the behavior of this query at all (Prometheus will still only pick values for `node_filesystem_files_free` and `node_filesystem_files` where all the labels match), but perhaps makes it cleaner to read. I'm extremely new to PromQL, and I'm mostly filing this as a learning opportunity. Are there best practices around this sort of thing? Do we like the way this was written, and should I change our other expressions ot match (some of the other expressions use `${diskSelector}` twice).