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: [email protected]
Fixes: 247b365 ("rust: add `kernel` crate")
Signed-off-by: David Gow <[email protected]>
Reviewed-by: Gary Guo <[email protected]>
Reviewed-by: Björn Roy Baron <[email protected]>
Reviewed-by: Vincenzo Palazzo <[email protected]>
[Applied `rustfmt`]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: sulix

Diff for: rust/kernel/print.rs

@@ -18,7 +18,11 @@ use crate::bindings;
 
 // 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()) };