rust: opp: use CStr::as_char_ptr

Replace the use of `as_ptr` which works through `<CStr as
Deref<Target=&[u8]>::deref()` in preparation for replacing
`kernel::str::CStr` with `core::ffi::CStr` as the latter does not
implement `Deref<Target=&[u8]>`.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://patch.msgid.link/20251018-cstr-core-v18-10-9378a54385f8@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: tamird

rust/kernel/opp.rs

@@ -13,7 +13,7 @@ use crate::{
     cpumask::{Cpumask, CpumaskVar},
     device::Device,
     error::{code::*, from_err_ptr, from_result, to_result, Result, VTABLE_DEFAULT_ERROR},
-    ffi::c_ulong,
+    ffi::{c_char, c_ulong},
     prelude::*,
     str::CString,
     sync::aref::{ARef, AlwaysRefCounted},
@@ -88,12 +88,12 @@ use core::{marker::PhantomData, ptr};
 use macros::vtable;
 
 /// Creates a null-terminated slice of pointers to [`Cstring`]s.
-fn to_c_str_array(names: &[CString]) -> Result<KVec<*const u8>> {
+fn to_c_str_array(names: &[CString]) -> Result<KVec<*const c_char>> {
     // Allocated a null-terminated vector of pointers.
     let mut list = KVec::with_capacity(names.len() + 1, GFP_KERNEL)?;
 
     for name in names.iter() {
-        list.push(name.as_ptr().cast(), GFP_KERNEL)?;
+        list.push(name.as_char_ptr(), GFP_KERNEL)?;
     }
 
     list.push(ptr::null(), GFP_KERNEL)?;