rust: fix signature of rust_fmt_argument

Without this change, the rest of this series will emit the following
error message:

error[E0308]: `if` and `else` have incompatible types
  --> <linux>/rust/kernel/print.rs:22:22
   |
21 | #[export]
   | --------- expected because of this
22 | unsafe extern "C" fn rust_fmt_argument(
   |                      ^^^^^^^^^^^^^^^^^ expected `u8`, found `i8`
   |
   = note: expected fn item `unsafe extern "C" fn(*mut u8, *mut u8, *mut c_void) -> *mut u8 {bindings::rust_fmt_argument}`
              found fn item `unsafe extern "C" fn(*mut i8, *mut i8, *const c_void) -> *mut i8 {print::rust_fmt_argument}`

The error may be different depending on the architecture.

To fix this, change the void pointer argument to use a const pointer,
and change the imports to use crate::ffi instead of core::ffi for
integer types.

Fixes: 787983d ("vsprintf: add new `%pA` format specifier")
Reviewed-by: Tamir Duberstein <tamird@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20250303-export-macro-v3-1-41fbad85a27f@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Darksonn

lib/vsprintf.c

@@ -2285,7 +2285,7 @@ int __init no_hash_pointers_enable(char *str)
 early_param("no_hash_pointers", no_hash_pointers_enable);
 
 /* Used for Rust formatting ('%pA'). */
-char *rust_fmt_argument(char *buf, char *end, void *ptr);
+char *rust_fmt_argument(char *buf, char *end, const void *ptr);
 
 /*
  * Show a '%p' thing.  A kernel extension is that the '%p' is followed

rust/kernel/print.rs

@@ -6,12 +6,11 @@
 //!
 //! Reference: <https://docs.kernel.org/core-api/printk-basics.html>
 
-use core::{
+use crate::{
     ffi::{c_char, c_void},
-    fmt,
+    str::RawFormatter,
 };
-
-use crate::str::RawFormatter;
+use core::fmt;
 
 // Called from `vsprintf` with format specifier `%pA`.
 #[expect(clippy::missing_safety_doc)]