rust: disallow use of `CStr::as_ptr` and `CStr::from_ptr`

rust: disallow use of `CStr::as_ptr` and `CStr::from_ptr`

As kernel always use unsigned char and not the platform ABI's default, an
user should always use `as_char_ptr` provided via `CStrExt` instead.
Therefore configure `disallow-methods` feature of clippy to catch incorrect
usage.

Similarly, the dual `from_ptr` is also disallowed.

[ As an example, without the previous commit, we would get a warning like:

      warning: use of a disallowed method `core::ffi::CStr::as_ptr`
         --> rust/kernel/task.rs:422:54
          |
      422 |         unsafe { crate::bindings::__might_sleep(file.as_ptr().cast(), loc.line() as i32) }
          |                                                      ^^^^^^ help: kernel's `char` is always unsigned, use `as_char_ptr` instead: `kernel::prelude::CStrExt::as_char_ptr`
          |
          = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.94.0/index.html#disallowed_methods
          = note: `-W clippy::disallowed-methods` implied by `-W clippy::all`
          = help: to override `-W clippy::all` add `#[allow(clippy::disallowed_methods)]`

    - Miguel ]

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Link: https://patch.msgid.link/20260203130745.868762-2-gary@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>