rust: macros: enable the rest of the tests

Now that the rusttest target for the macros crate is compiled with the
kernel crate as a dependency, the rest of the rustdoc tests can be
enabled.

Signed-off-by: Ethan D. Twardy <ethan.twardy@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://github.com/Rust-for-Linux/linux/issues/1076
Link: https://lore.kernel.org/r/20240704145607.17732-5-ethan.twardy@gmail.com


[ Rebased (use `K{Box,Vec}` instead, enable `lint_reasons` feature).
  Remove unneeded `rust` as language in examples, as well as
  `#[macro_use]` `extern`s. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>