With default options, when we pass --config-net, the IPv6 address is
actually going to be recycled from the init namespace, so it is in
fact duplicated, but duplicate address detection has no way to find
out.
With a different configured address, that's not the case, but anyway
duplicate address detection will be unable to see this.
In both cases, we're wasting time for nothing.
Pass the IFA_F_NODAD flag as we configure globally scoped IPv6
addresses via netlink.
Signed-off-by: Stefano Brivio