net/ethernet.h: Add make_ether_addr()

[ivy]

Summary

This function turns an array of octets into a struct ether_addr,
which is useful when type-safe code (which uses ether_addr) wants
to interoperate with the many parts of the kernel where addresses
are stored as plain byte arrays.

To avoid sacrificing performance for safety, make the function
static inline.

[ivy]

this came up while hacking on if_bridge but i thought it might be more generally useful.

[zlei]

The struct ether_addr is a thin wrapper of ETHER_ADDR_LEN octets. A memcpy should be simple and enough, so what's the value of this inline function ?

[hrs]

Could you explain why a simple type casting is not enough?