rnull: replace kernel::c_str! with C-Strings

C-String literals were added in Rust 1.77. Replace instances of
`kernel::c_str!` with C-String literals where possible.

Signed-off-by: Tamir Duberstein <[email protected]>
Reviewed-by: Daniel Almeida <[email protected]>

Author: tamird

drivers/block/rnull/configfs.rs

@@ -25,7 +25,7 @@ pub(crate) fn subsystem() -> impl PinInit<kernel::configfs::Subsystem<Config>, E
         ],
     };
 
-    kernel::configfs::Subsystem::new(c_str!("rnull"), item_type, try_pin_init!(Config {}))
+    kernel::configfs::Subsystem::new(c"rnull", item_type, try_pin_init!(Config {}))
 }
 
 #[pin_data]