selftests: mptcp: add testcases for removing addrs

This patch added the testcases for removing a list of addresses. Used
the netlink to flush the addresses in the testcases.

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