Categorize all the lints!

clippy_lints/src/mut_reference.rs

@@ -16,9 +16,9 @@ use utils::span_lint;
 /// ```rust
 /// my_vec.push(&mut value)
 /// ```
-declare_lint! {
+declare_clippy_lint! {
     pub UNNECESSARY_MUT_PASSED,
-    Warn,
+    style,
     "an argument passed as a mutable reference although the callee only demands an \
      immutable reference"
 }