after apt upgrade wireguard is unable to build kernel module

we tried to build old version of wireguard-dkms 1.0.20200520, it builds without error.

Getting the same error on (Ubuntu 16.04 LTS, Xenial). /var/crash/wireguard-dkms.0.crash contents below:

```
ProblemType: Package
DKMSBuildLog:
 DKMS make.log for wireguard-1.0.20200611 for kernel 4.4.0-177-generic (x86_64)
 Fri Jun 12 20:09:26 EDT 2020
 make: Entering directory '/usr/src/linux-headers-4.4.0-177-generic'
   LD /var/lib/dkms/wireguard/1.0.20200611/build/built-in.o
   CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/main.o
   CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/noise.o
   CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/device.o
   CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/peer.o
   CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/timers.o
   CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/queueing.o
   CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/send.o
   CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/receive.o
   CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/socket.o
 /var/lib/dkms/wireguard/1.0.20200611/build/socket.c: In function ‘send6’:
 /var/lib/dkms/wireguard/1.0.20200611/build/socket.c:139:18: error: ‘const struct ipv6_stub’ has no member named ‘ipv6_dst_lookup_flow’
    dst = ipv6_stub->ipv6_dst_lookup_flow(sock_net(sock), sock, &fl,
                   ^
 scripts/Makefile.build:285: recipe for target '/var/lib/dkms/wireguard/1.0.20200611/build/socket.o' failed
 make[1]: *** [/var/lib/dkms/wireguard/1.0.20200611/build/socket.o] Error 1
 Makefile:1457: recipe for target '_module_/var/lib/dkms/wireguard/1.0.20200611/build' failed
 make: *** [_module_/var/lib/dkms/wireguard/1.0.20200611/build] Error 2
 make: Leaving directory '/usr/src/linux-headers-4.4.0-177-generic'
DKMSKernelVersion: 4.4.0-177-generic
Date: Fri Jun 12 20:09:36 2020
DuplicateSignature: dkms:wireguard-dkms:1.0.20200611-0ppa1~16.04:/var/lib/dkms/wireguard/1.0.20200611/build/socket.c:139:18: error: ‘const struct ipv6_stub’ has no member named ‘ipv6_dst_lookup_flow’
Package: wireguard-dkms 1.0.20200611-0ppa1~16.04
PackageVersion: 1.0.20200611-0ppa1~16.04
SourcePackage: wireguard-linux-compat
Title: wireguard-dkms 1.0.20200611-0ppa1~16.04: wireguard kernel module failed to build
```

Status changed to 'Confirmed' because the bug affects multiple users.

Having the same problem:
DKMS make.log for wireguard-1.0.20200611 for kernel 4.4.0-128-generic (x86_64)
Sun Aug 30 18:55:58 CEST 2020
make: Entering directory '/usr/src/linux-headers-4.4.0-128-generic'
  LD /var/lib/dkms/wireguard/1.0.20200611/build/built-in.o
  CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/main.o
  CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/noise.o
  CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/device.o
  CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/peer.o
  CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/timers.o
  CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/queueing.o
  CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/send.o
  CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/receive.o
  CC [M] /var/lib/dkms/wireguard/1.0.20200611/build/socket.o
/var/lib/dkms/wireguard/1.0.20200611/build/socket.c: In function ‘send6’:
/var/lib/dkms/wireguard/1.0.20200611/build/socket.c:139:18: error: ‘const struct ipv6_stub’ has no member named ‘ipv6_dst_lookup_flow’
   dst = ipv6_stub->ipv6_dst_lookup_flow(sock_net(sock), sock, &fl,
                  ^
scripts/Makefile.build:277: recipe for target '/var/lib/dkms/wireguard/1.0.20200611/build/socket.o' failed
make[1]: *** [/var/lib/dkms/wireguard/1.0.20200611/build/socket.o] Error 1
Makefile:1437: recipe for target '_module_/var/lib/dkms/wireguard/1.0.20200611/build' failed
make: *** [_module_/var/lib/dkms/wireguard/1.0.20200611/build] Error 2

This looks like a duplicate of bug #1862413.