We currently build CONFIG_IPV6=m.

This seems to be not really well-supported in mainline kernels - see
https://lore.kernel.org/netdev/20201115224509.2020651-1-flokli@flokli.de/T/#u

Compiling it as a module doesn't give too much benefit - even for people
who did explicitly set enableIPv6 to false, the ipv6 module was
still loaded, as soon as another module was loaded that requires it
(bridge,br_netfilter,wireguard,ip6table_mangle,sctp,…).

By compiling it in, we only loose the possibility to not add it to
boot.kernelModules anymore (as it's part of the kernel directly). The
space savings are negligible.

People wanting to disable IPv6 still get the appropriate sysctls and
options set (while having the kernel code loaded), nothing is really
changing here.

Motivation for this change

ip route add 1.2.3.4/32 via inet6 fe80::1 dev eth0

fails with the message "Error: IPv6 support not enabled in kernel." if
CONFIG_IPV6 is m.

Things done

I poked one of the existing networking tests, and tried to program the route manually.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.