rust: kernel: Mark rust_fmt_argument as extern "C"

The rust_fmt_argument function is called from printk() to handle the %pA
format specifier.

Since it's called from C, we should mark it extern "C" to make sure it's
ABI compatible.

Cc: stable@vger.kernel.org
Fixes: 247b365d ("rust: add `kernel` crate")
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
[Applied `rustfmt`]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/print.rs

@@ -18,7 +18,11 @@
 
 // Called from `vsprintf` with format specifier `%pA`.
 #[no_mangle]
-unsafe fn rust_fmt_argument(buf: *mut c_char, end: *mut c_char, ptr: *const c_void) -> *mut c_char {
+unsafe extern "C" fn rust_fmt_argument(
+    buf: *mut c_char,
+    end: *mut c_char,
+    ptr: *const c_void,
+) -> *mut c_char {
     use fmt::Write;
     // SAFETY: The C contract guarantees that `buf` is valid if it's less than `end`.
     let mut w = unsafe { RawFormatter::from_ptrs(buf.cast(), end.cast()) };