[PATCH] rust/fs: use intra-doc link for `EBADF` in `BadFdError` docs

[Tong]

rust/fs: use intra-doc link for `EBADF` in `BadFdError` docs

This patch changes the documentation for `BadFdError` in the `fs`
module so that the error variant `EBADF` is referenced via an
intra-doc link (i.e., [`EBADF`]) rather than plain text. This improves
the generated documentation (so the `EBADF` constant is linked), and
helps tools like rust-analyzer to resolve the reference from the docs.

Testing:
- regenerated documentation, confirmed the link to `EBADF` shows up correctly,
- ran doctests to ensure no broken links in docs,
- built affected code with rustfmt / clippy to verify formatting and
lint cleanliness.

Suggested-by: Onur Özkan <work@xxxxxxxxxxxxx>
Link: https://github.com/Rust-for-Linux/linux/issues/1186
Signed-off-by: djfkvcing117@xxxxxxxxx

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