rust: configfs: use `core::ffi::CStr` method names

Prepare for `core::ffi::CStr` taking the place of `kernel::str::CStr` by
avoid methods that only exist on the latter.

Also avoid `Deref<Target=BStr> for CStr` as that impl doesn't exist on
`core::ffi::CStr`.

Link: https://github.com/Rust-for-Linux/linux/issues/1075
Acked-by: Greg Kroah-Hartman <[e-mail]>
Reviewed-by: Alice Ryhl <[e-mail]>
Reviewed-by: Benno Lossin <[e-mail]>
Signed-off-by: Tamir Duberstein <[e-mail]>
Acked-by: Andreas Hindborg <[e-mail]>
Signed-off-by: Miguel Ojeda <[e-mail]>

rust/kernel/configfs.rs

···
         try_pin_init!(Self {
             group <- pin_init::init_zeroed().chain(|v: &mut Opaque<bindings::config_group>| {
                 let place = v.get();
-                let name = name.as_bytes_with_nul().as_ptr();
+                let name = name.to_bytes_with_nul().as_ptr();
                 // SAFETY: It is safe to initialize a group once it has been zeroed.
                 unsafe {
                     bindings::config_group_init_type_name(place, name.cast(), item_type.as_ptr())
···
     pub const fn new(name: &'static CStr) -> Self {
         Self {
             attribute: Opaque::new(bindings::configfs_attribute {
-                ca_name: name.as_char_ptr(),
+                ca_name: crate::str::as_char_ptr_in_const_context(name),
                 ca_owner: core::ptr::null_mut(),
                 ca_mode: 0o660,
                 show: Some(Self::show),

samples/rust/rust_configfs.rs

···
 
     fn show(container: &Configuration, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
         pr_info!("Show message\n");
-        let data = container.message;
+        let data = container.message.to_bytes();
         page[0..data.len()].copy_from_slice(data);
         Ok(data.len())
     }