selftests: mptcp: remove id 0 address testcases

This patch added the testcases for removing the id 0 subflow and the id 0
address.

In do_transfer, use the removing addresses number '9' for deleting the id
0 address.

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>