wireguard: bump to 0.0.20200318

wireguard: bump to 0.0.20200318

WireGuard had a brief professional security audit. The auditors didn't find
any vulnerabilities, but they did suggest one defense-in-depth suggestion to
protect against potential API misuse down the road, mentioned below. This
compat snapshot corresponds with the patches I just pushed to Dave for
5.6-rc7.

* curve25519-x86_64: avoid use of r12

This buys us 100 extra cycles, which isn't much, but it winds up being even
faster on PaX kernels, which use r12 as a RAP register.

* wireguard: queueing: account for skb->protocol==0

This is the defense-in-depth change. We deal with skb->protocol==0 just fine,
but the advice to deal explicitly with it seems like a good idea.

* receive: remove dead code from default packet type case

A default case of a particular switch statement should never be hit, so
instead of printing a pretty debug message there, we full-on WARN(), so that
we get bug reports.

* noise: error out precomputed DH during handshake rather than config

All peer keys will now be addable, even if they're low order. However, no
handshake messages will be produced successfully. This is a more consistent
behavior with other low order keys, where the handshake just won't complete if
they're being used anywhere.

* send: use normaler alignment formula from upstream

We're trying to keep a minimal delta with upstream for the compat backport.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>