rust: uaccess: take advantage of the prelude and Result's defaults

The `kernel` prelude brings `Result` and the error codes; and the prelude
itself is already available in the examples automatically.

In addition, `Result` already defaults to `T = ()`.

Thus simplify.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250429151445.438977-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/uaccess.rs

@@ -46,10 +46,9 @@ pub type UserPtr = usize;
 ///
 /// ```no_run
 /// use kernel::ffi::c_void;
-/// use kernel::error::Result;
 /// use kernel::uaccess::{UserPtr, UserSlice};
 ///
-/// fn bytes_add_one(uptr: UserPtr, len: usize) -> Result<()> {
+/// fn bytes_add_one(uptr: UserPtr, len: usize) -> Result {
 ///     let (read, mut write) = UserSlice::new(uptr, len).reader_writer();
 ///
 ///     let mut buf = KVec::new();
@@ -68,7 +67,6 @@ pub type UserPtr = usize;
 ///
 /// ```no_run
 /// use kernel::ffi::c_void;
-/// use kernel::error::{code::EINVAL, Result};
 /// use kernel::uaccess::{UserPtr, UserSlice};
 ///
 /// /// Returns whether the data in this region is valid.