Two issues were highlighted as a result of using either Void Linux with disk encryption, or just disk encryption on Linux in general:
Two fixes:
1. Fixes a failed `usage()` call in the `get_disk_usage` function from failing the entire result. Now it only returns an entry with N/A results. This occurred in some distros and disk encryption setups, for example, the one for Void Linux here: https://docs.voidlinux.org/installation/guides/fde.html.
2. Fixes a potential mapping issue with disk encryption on Linux in general. Since the disk might map to `/dev/mapper/whatever`, but the I/O harvester was using another name, the mappings would not match. As such, we now also check if a symlink exists; if it does, then we take it and work out the correct path. This also fixes the disk name being wrong.
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