selftests: mptcp: add ADD_ADDR IPv6 test cases

This patch added IPv6 support for do_transfer, and the test cases for
ADD_ADDR IPv6.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>