wireguard: bump to 0.0.20190702

wireguard: bump to 0.0.20190702

* curve25519: not all linkers support bmi2 and adx

This should allow WireGuard to build on older toolchains.

* global: switch to coarse ktime

Our prior use of fast ktime before meant that sometimes, depending on how
broken the motherboard was, we'd wind up calling into the HPET slow path. Here
we move to coarse ktime which is always super speedy. In the process we had to
fix the resolution of the clock, as well as introduce a new interface for it,
landing in 5.3. Older kernels fall back to a fast-enough mechanism based on
jiffies.

https://lore.kernel.org/lkml/tip-e3ff9c3678b4d80e22d2557b68726174578eaf52@git.kernel.org/
https://lore.kernel.org/lkml/20190621203249.3909-3-Jason@zx2c4.com/

* netlink: cast struct over cb->args for type safety

This follow recent upstream changes such as:

https://lore.kernel.org/lkml/20190628144022.31376-1-Jason@zx2c4.com/

* peer: use LIST_HEAD macro

Style nit.

* receive: queue dead packets to napi queue instead of empty rx_queue

This mitigates a WARN_ON being triggered by the workqueue code. It was quite
hard to trigger, except sporadically, or reliably with a PC Engines ALIX, an
extremely slow board with an AMD LX800 that Ryan Whelan of Axatrax was kind
enough to mail me.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
(cherry picked from commit 7c23f741e97f6645bb5cd662a4943796a344b26a)