rust: prelude: add error::code::* constant items

It is convenient to have all the `Error` constant items (such as
`EINVAL`) available as-is everywhere (i.e. for code using the kernel
prelude such as kernel modules).

Therefore, add all of them to the prelude.

For instance, this allows to write `Err(EINVAL)` to create
a kernel `Result`:

    fn f() -> Result<...> {
        ...
        Err(EINVAL)
    }

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/prelude.rs

@@ -19,6 +19,6 @@ pub use macros::{module, vtable};
 
 pub use super::{pr_alert, pr_crit, pr_debug, pr_emerg, pr_err, pr_info, pr_notice, pr_warn};
 
-pub use super::error::{Error, Result};
+pub use super::error::{code::*, Error, Result};
 
 pub use super::ThisModule;