rust: kernel: str: Implement Debug for CString

Make it possible to use a `CString` with the `pr_*` macros directly. That
is, instead of:

    pr_debug!("trying to open {:?}\n", &*filename);

we can now write:

    pr_debug!("trying to open {:?}\n", filename);

Signed-off-by: Asahi Lina <lina@asahilina.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20230714-cstring-debug-v1-1-4e7c3018dd4f@asahilina.net
[ Reworded to use Alice's commit message as discussed. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: hoshinolina

rust/kernel/str.rs

@@ -608,6 +608,12 @@ impl<'a> TryFrom<&'a CStr> for CString {
     }
 }
 
+impl fmt::Debug for CString {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&**self, f)
+    }
+}
+
 /// A convenience alias for [`core::format_args`].
 #[macro_export]
 macro_rules! fmt {