rust/kernel: remove Error::from_kernel_errno() dead code warning

It turns out that we don't need an `Error` constructor that
checks the `errno` invariant at runtime. This is because we
trust return values originating from kernel C. And any return
values originating from Rust code can be constucted using
`Error::` constants, which also do not need a runtime check:

```rust
    return Err(Error::EBUSY);
```

However, there is still ongoing discussion about the merits and
purpose of this function. To facilitate this discussion, keep the
function for the time being, and remove the "dead code" warning.

Signed-off-by: Sven Van Asbroeck <[email protected]>

rust/kernel/error.rs

@@ -64,6 +64,8 @@ impl Error {
     ///
     /// It is a bug to pass an out-of-range `errno`. `EINVAL` would
     /// be returned in such a case.
+    // TODO: remove `dead_code` marker once an in-kernel client is available.
+    #[allow(dead_code)]
     pub(crate) fn from_kernel_errno(errno: c_types::c_int) -> Error {
         if errno < -(bindings::MAX_ERRNO as i32) || errno >= 0 {
             // TODO: make it a `WARN_ONCE` once available.