rust: bitmap: fix formatting

We do our best to keep the repository `rustfmt`-clean, thus run the tool
to fix the formatting issue.

Link: https://docs.kernel.org/rust/coding-guidelines.html#style-formatting
Link: https://rust-for-linux.com/contributing#submit-checklist-addendum


Fixes: 0f587883 ("rust: bitmap: clean Rust 1.92.0 `unused_unsafe` warning")
Reviewed-by: Burak Emir <bqe@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>