kernel config: explicitly enable CONFIG_IPV6

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.

nixos/modules/tasks/network-interfaces.nix

@@ -1062,7 +1062,6 @@ in
       ];
 
     boot.kernelModules = [ ]
-      ++ optional cfg.enableIPv6 "ipv6"
       ++ optional hasVirtuals "tun"
       ++ optional hasSits "sit"
       ++ optional hasBonds "bonding";

pkgs/os-specific/linux/kernel/common-config.nix

@@ -132,6 +132,7 @@ let
       IP_MROUTE_MULTIPLE_TABLES   = yes;
       IP_MULTICAST                = yes;
       IP_MULTIPLE_TABLES          = yes;
+      IPV6                        = yes;
       IPV6_ROUTER_PREF            = yes;
       IPV6_ROUTE_INFO             = yes;
       IPV6_OPTIMISTIC_DAD         = yes;