selftests: mptcp: add cfg_do_w for cfg_remove

In some testcases, we need to slow down the transmitting process. This
patch added a new argument named cfg_do_w for cfg_remove to allow the
caller to pass an argument to cfg_remove.

In do_rnd_write, use this cfg_do_w to control the transmitting speed.

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>