rust: error: declare errors using macro

Add a macro to declare errors, which simplifies the work needed to
add each one, avoids repetition of the code and makes it easier to
change the way they are declared.
Signed-off-by: Finn Behrens <me@kloenk.dev>
Reviewed-by: Gary Guo <gary@garyguo.net>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/error.rs

@@ -8,8 +8,16 @@
 
 /// Contains the C-compatible error codes.
 pub mod code {
-    /// Out of memory.
-    pub const ENOMEM: super::Error = super::Error(-(crate::bindings::ENOMEM as i32));
+    macro_rules! declare_err {
+        ($err:tt $(,)? $($doc:expr),+) => {
+            $(
+            #[doc = $doc]
+            )*
+            pub const $err: super::Error = super::Error(-(crate::bindings::$err as i32));
+        };
+    }
+
+    declare_err!(ENOMEM, "Out of memory.");
 }
 
 /// Generic integer kernel error.