bug: Fix bugs with disk widget and disk encryption

[ClementTsang]

Description

A description of the change and what it does. If relevant, please provide screenshots of what results from the change:

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.
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.

Void Linux + LUKS:

PopOS with disk encryption:

Issue

If applicable, what issue does this address?

#419

Type of change

Remove the irrelevant ones:

• Bug fix (non-breaking change which fixes an issue)

Test methodology

If relevant, please state how this was tested:

Furthermore, please tick which platforms this change was tested on:

• Windows
• macOS
• Linux

If relevant, all of these platforms should be tested.

Checklist

If relevant, ensure the following have been met:

• Change has been tested to work, and does not cause new breakage unless intended
• Code has been self-reviewed
• Documentation has been added/updated if needed (README, help menu, etc.)
• Passes CI pipeline (clippy check and cargo test check)
• Areas your change affects have been linted using rustfmt (cargo fmt)
• No merge conflicts arise from the change

Other information

Provide any other relevant information to this change:

[ClementTsang]

...please ignore the commit history, I was lazy and so did my debugging tests using git to sync my main machine and my VM...