Kernel: Fix handful of remaining "return -EFOO" mistakes

Now that all KResult and KResultOr are used consistently throughout the
kernel, it's no longer necessary to return negative error codes.
However, we were still doing that in some places, so let's fix all those
(bugs) by removing the minuses. :^)

Kernel/FileSystem/Plan9FileSystem.cpp

@@ -618,7 +618,7 @@ KResult Plan9FS::post_message_and_wait_for_a_reply(Message& message)
         return KResult((ErrnoCode)error_code);
     } else if (reply_type == Message::Type::Rerror) {
         // Contains an error message. We could attempt to parse it, but for now
-        // we simply return -EIO instead. In 9P200.u, it can also contain a
+        // we simply return EIO instead. In 9P200.u, it can also contain a
         // numerical errno in an unspecified encoding; we ignore those too.
         StringView error_name;
         message >> error_name;