rust: kernel: fix unused_mut warnings

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

rust/kernel/chrdev.rs

@@ -160,7 +160,7 @@ impl<const N: usize> Registration<{ N }> {
             });
         }
 
-        let mut inner = this.inner.as_mut().unwrap();
+        let inner = this.inner.as_mut().unwrap();
         if inner.used == N {
             return Err(EINVAL);
         }

rust/kernel/fs.rs

@@ -428,7 +428,7 @@ impl Registration {
             return Err(EINVAL);
         }
 
-        let mut fs = this.fs.get_mut();
+        let fs = this.fs.get_mut();
         fs.owner = module.0;
         fs.name = T::NAME.as_char_ptr();
         fs.fs_flags = T::FLAGS;