rust: file: add intra-doc link for 'EBADF'

The `BadFdError` doc comment mentions the `EBADF` constant but does
not currently provide a navigation target for readers of the
generated docs. Turning the references into intra-doc links matches
the rest of the module and makes the documentation easier to
explore.

Suggested-by: Onur Özkan <work@onurozkan.dev>
Link: https://github.com/Rust-for-Linux/linux/issues/1186


Signed-off-by: Tong Li <djfkvcing117@gmail.com>
Reviewed-by: Onur Özkan <work@onurozkan.dev>
Signed-off-by: Christian Brauner <brauner@kernel.org>