rust: error: make conversion functions public

Change visibility to public of functions in error.rs:
from_err_ptr, from_errno, from_result and to_ptr.
Additionally, remove dead_code annotations.

Link: https://github.com/Rust-for-Linux/linux/issues/1105


Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Filipe Xavier <felipe_life@live.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/DM4PR14MB7276E6948E67B3B23D8EA847E9652@DM4PR14MB7276.namprd14.prod.outlook.com


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>