nixos/network-interfaces-scripted: disable DAD

Duplicate address detection (DAD) causes newly added IPv6 addresses to
be effectively unusable for an unpredictable amount of time, introducing
race conditions in the network setup.
For example, a "tentative" addresses is not considered a valid source
address, so installing routes can fail unpredictably.

This change disables DAD for static IPv6 addresses, with the assumption
that the user already made sure they are unique when configuring the
network.

nixos/modules/tasks/network-interfaces-scripted.nix

@@ -240,10 +240,10 @@ let
                 ''
                   echo "${cidr}" >> $state
                   echo -n "adding address ${cidr}... "
-                  if out=$(ip addr replace "${cidr}" dev "${i.name}" 2>&1); then
+                  if out=$(ip addr replace "${cidr}" dev "${i.name}" nodad 2>&1); then
                     echo "done"
                   else
-                    echo "'ip addr replace \"${cidr}\" dev \"${i.name}\"' failed: $out"
+                    echo "'ip addr replace \"${cidr}\" dev \"${i.name}\"' nodad failed: $out"
                     exit 1
                   fi
                 ''