Add ethernet lengths and types, and header

This patch adds a module to the `net` module of `kernel` that provides
constants for ethernet lengths (address length, type length, etc), as
well as the enumeration of all protocols currently understood by the
network stack. The latter are represented as 16-bit integers in memory
to simplify conversion to and from the types read directly from Ethernet
II frames. This patch also contains a conversion from/to `u16`.

The `ethernet` module also contains a `Header` abstraction that just
wraps around `struct ethhdr` defined in the UAPI. There is a simple
builder function implemented as well as getters for src/dst address, and
protocol number.

Signed-off-by: Amélie Gonzalez <[email protected]>

rust/kernel/net.rs

@@ -9,6 +9,7 @@
 use crate::{bindings, str::CStr, to_result, ARef, AlwaysRefCounted, Error, Result};
 use core::{cell::UnsafeCell, ptr::NonNull};
 
+pub mod ethernet;
 #[cfg(CONFIG_NETFILTER)]
 pub mod filter;