Fix dead_code in rust KParamGuard.

Signed-off-by: Finn Behrens <me@kloenk.de>

Author: kloenk

rust/kernel/lib.rs

@@ -131,15 +131,25 @@ impl ThisModule {
             bindings::kernel_param_lock(self.0)
         }
 
-        KParamGuard { this_module: self }
+        #[cfg(CONFIG_SYSFS)]
+        return KParamGuard { this_module: self };
+
+        #[cfg(not(CONFIG_SYSFS))]
+        return KParamGuard { _private: () };
     }
 }
 
 /// Scoped lock on the kernel parameters of [`ThisModule`].
 ///
 /// Lock will be released when this struct is dropped.
+#[cfg_attr(CONFIG_SYSFS, repr(transparent))]
 pub struct KParamGuard<'a> {
+    #[cfg(CONFIG_SYSFS)]
     this_module: &'a ThisModule,
+
+    // private field, so this struct can not be constructed from outside this crate
+    #[cfg(not(CONFIG_SYSFS))]
+    _private: (),
 }
 
 #[cfg(CONFIG_SYSFS)]