selftests: mptcp: fix ipv6 routing setup

commit 9846921d upstream.

MPJ ipv6 selftests currently lack per link route to the server
net. Additionally, ipv6 subflows endpoints are created without any
interface specified. The end-result is that in ipv6 self-tests
subflows are created all on the same link, leading to expected delays
and sporadic self-tests failures.

Fix the issue by adding the missing setup bits.

Fixes: 523514ed ("selftests: mptcp: add ADD_ADDR IPv6 test cases")
Reported-and-tested-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>