Add ethernet lengths and types, and header

This patch adds a module to the `net` module of `kernel` that provides
enumerations with 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 <lymkwi@vulpinecitrus.info>

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;