rust: fmt: fix formatting expressions

rust: fmt: fix formatting expressions

Allow usage like `pr_info!("one + 1 = {}", one + 1)` to compile by
ensuring that a reference is taken to the entire expression.

[ The errors we would get otherwise look like:

    error[E0277]: `kernel::fmt::Adapter<i32>` doesn't implement `core::fmt::Display`
      --> ../samples/rust/rust_minimal.rs:34:9
       |
    34 |         pr_info!("one + 1 = {}", one + 1);
       |         ^^^^^^^^^^^^^^^^^^^^--^^^^^^^^^^^
       |         |                   |
       |         |                   required by this formatting parameter
       |         `kernel::fmt::Adapter<i32>` cannot be formatted with the default formatter
       |
       = help: the trait `core::fmt::Display` is not implemented for `kernel::fmt::Adapter<i32>`
       = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
       = help: the trait `core::fmt::Display` is implemented for `kernel::fmt::Adapter<&T>`
       = note: this error originates in the macro `$crate::print_macro` which comes from the expansion of the macro `pr_info` (in Nightly builds, run with -Z macro-backtrace for more info)

    - Miguel ]

Fixes: c5cf01ba8dfe ("rust: support formatting of foreign types")
Reported-by: Janne Grunau <j@jannau.net>
Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Custom.20formatting/near/566219493
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Janne Grunau <j@jannau.net>
Reviewed-by: Janne Grunau <j@jannau.net>
Link: https://patch.msgid.link/20260104-fmt-paren-v1-1-6b84bc0da78f@gmail.com
[ Added Signed-off-by back. Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>