selftests: mptcp: add the flush addrs testcase

This patch added the flush addrs testcase. In do_transfer, if the number
of removing addresses is less than 8, use the del addr command to remove
the addresses one by one. If the number is more than 8, use the flush addrs
command to remove the addresses.

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>