selftests: mptcp: add addr argument for del_addr

For the id 0 address, different MPTCP connections could be using
different IP addresses for id 0.

This patch added an extra argument IP address for del_addr when
using id 0.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>