rust: init: use `#[default_error(err)]` for the initializer macros

rust: init: use `#[default_error(err)]` for the initializer macros

Initializer macros should use this attribute instead of manually parsing
the macro's input. This is because the syntax is now parsed using `syn`,
which permits more complex constructs to be parsed. In addition, this
ensures that the kernel's initializer marcos will have the exact same
syntax as the ones from pin-init.

Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <lossin@kernel.org>