rust: use `kernel::{fmt,prelude::fmt!}`

rust: use `kernel::{fmt,prelude::fmt!}`

Reduce coupling to implementation details of the formatting machinery by
avoiding direct use for `core`'s formatting traits and macros.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250704-core-cstr-prepare-v1-3-a91524037783@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>