rust: kbuild: give `--config-path` to `rustfmt` in `.rsi` target

`rustfmt` is configured via the `.rustfmt.toml` file in the source tree,
and we apply `rustfmt` to the macro expanded sources generated by the
`.rsi` target.

However, under an `O=` pointing to an external folder (i.e. not just
a subdir), `rustfmt` will not find the file when checking the parent
folders. Since the edition is configured in this file, this can lead to
errors when it encounters newer syntax, e.g.

error: expected one of `!`, `.`, `::`, `;`, `?`, `where`, `{`, or an operator, found `"rust_minimal"`
--> samples/rust/rust_minimal.rsi:29:49
|
28 | impl ::kernel::ModuleMetadata for RustMinimal {
| - while parsing this item list starting here
29 | const NAME: &'static ::kernel::str::CStr = c"rust_minimal";
| ^^^^^^^^^^^^^^ expected one of 8 possible tokens
30 | }
| - the item list ends here
|
= note: you may be trying to write a c-string literal
= note: c-string literals require Rust 2021 or later
= help: pass `--edition 2024` to `rustc`
= note: for more on editions, read https://doc.rust-lang.org/edition-guide

A workaround is to use `RUSTFMT=n`, which is documented in the `Makefile`
help for cases where macro expanded source may happen to break `rustfmt`
for other reasons, but this is not one of those cases.

One solution would be to pass `--edition`, but we want `rustfmt` to
use the entire configuration, even if currently we essentially use the
default configuration.

Thus explicitly give the path to the config file to `rustfmt` instead.

Reported-by: Alice Ryhl <aliceryhl@google.com>
Fixes: 2f7ab1267dc9 ("Kbuild: add Rust support")
Cc: stable@vger.kernel.org
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260115183832.46595-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>