rust: err: add __rust_helper to helpers

This is needed to inline these helpers into Rust code.

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260105-define-rust-helper-v2-8-51da5f454a67@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Darksonn

rust/helpers/build_bug.c

@@ -2,7 +2,7 @@
 
 #include <linux/errname.h>
 
-const char *rust_helper_errname(int err)
+__rust_helper const char *rust_helper_errname(int err)
 {
 	return errname(err);
 }

rust/helpers/err.c

@@ -2,17 +2,17 @@
 
 #include <linux/err.h>
 
-__force void *rust_helper_ERR_PTR(long err)
+__rust_helper __force void *rust_helper_ERR_PTR(long err)
 {
 	return ERR_PTR(err);
 }
 
-bool rust_helper_IS_ERR(__force const void *ptr)
+__rust_helper bool rust_helper_IS_ERR(__force const void *ptr)
 {
 	return IS_ERR(ptr);
 }
 
-long rust_helper_PTR_ERR(__force const void *ptr)
+__rust_helper long rust_helper_PTR_ERR(__force const void *ptr)
 {
 	return PTR_ERR(ptr);
 }