wireguard: bump to 1.0.20200520

wireguard: bump to 1.0.20200520

This version has the various slew of bug fixes and compat fixes and
such, but the most interesting thing from an OpenWRT perspective is that
WireGuard now plays nicely with cake and fq_codel. I'll be very
interested to hear from OpenWRT users whether this makes a measurable
difference. Usual set of full changes follows.

This release aligns with the changes I sent to DaveM for 5.7-rc7 and were
pushed to net.git about 45 minutes ago.

* qemu: use newer iproute2 for gcc-10
* qemu: add -fcommon for compiling ping with gcc-10

These enable the test suite to compile with gcc-10.

* noise: read preshared key while taking lock

Matt noticed a benign data race when porting the Linux code to OpenBSD.

* queueing: preserve flow hash across packet scrubbing
* noise: separate receive counter from send counter

WireGuard now works with fq_codel, cake, and other qdiscs that make use of
skb->hash. This should significantly improve latency spikes related to
buffer bloat. Here's a before and after graph from some data Toke measured:
https://data.zx2c4.com/removal-of-buffer-bloat-in-wireguard.png

* compat: support RHEL 8 as 8.2, drop 8.1 support
* compat: support CentOS 8 explicitly
* compat: RHEL7 backported the skb hash renamings

The usual RHEL churn.

* compat: backport renamed/missing skb hash members

The new support for fq_codel and friends meant more backporting work.

* compat: ip6_dst_lookup_flow was backported to 4.14, 4.9, and 4.4

The main motivation for releasing this now: three stable kernels were released
at the same time, with a patch that necessitated updating in our compat layer.

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