selftests: seg6: disable DAD on IPv6 router cfg for srv6_end_dt4_l3vpn_test

The srv6_end_dt4_l3vpn_test instantiates a virtual network consisting of
several routers (rt-1, rt-2) and hosts.
When the IPv6 addresses of rt-{1,2} routers are configured, the Deduplicate
Address Detection (DAD) kicks in when enabled in the Linux distros running
the selftests. DAD is used to check whether an IPv6 address is already
assigned in a network. Such a mechanism consists of sending an ICMPv6 Echo
Request and waiting for a reply.
As the DAD process could take too long to complete, it may cause the
failing of some tests carried out by the srv6_end_dt4_l3vpn_test script.

To make the srv6_end_dt4_l3vpn_test more robust, we disable DAD on routers
since we configure the virtual network manually and do not need any address
deduplication mechanism at all.

Fixes: 2195444e ("selftests: add selftest for the SRv6 End.DT4 behavior")
Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>