ls.rs: Rename a variable

[sylvestre]

in the ls code, we have a variable called dfn.

https://github.com/uutils/coreutils/blob/main/src/uu/ls/src/ls.rs#L2485

It should be renamed for something like
displayed_name or something less cryptic

Originally posted by @sylvestre in #5280 (comment)