wireguard: problem sending via libpcap's packet socket

[Jason A. Donenfeld]

Hi again Hans,

A few remarks: although gre implements header_ops, it looks like
various parts of the networking stack change behavior based on it. I'm
still analyzing that to understand the extent of the effects.
Something like <https://git.zx2c4.com/wireguard-linux/commit/?id=40c24fd379edc1668087111506ed3d0928052fe0>
would work, but I'm not thrilled by it. Further research is needed.

However, one thing I noticed is that other layer 3 tunnels don't seem
to be a fan of libpcap. For example, try injecting a packet into an
ipip interface. You'll hit exactly the same snag for skb->protocol==0.
So, if I do go the route of the first option -- adding a header_ops --
maybe I'll be inclined to make a shared l3_header_ops struct that can
be shared between things, and fix up all of these at once.

Alternatively, it might turn out to be that, because this is broken
for other layer 3 devices, it's meant to be broken here. But I hope
that won't be the conclusion.

Jason