selftests: mptcp: add remove cfg in mptcp_connect

This patch added a new cfg, named cfg_remove in mptcp_connect. This new
cfg_remove is copied from cfg_join. The only difference between them is in
the do_rnd_write function. Here we slow down the transfer process of all
data to let the RM_ADDR suboption can be sent and received completely.
Otherwise the remove address and subflow test cases don't work.

Suggested-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Suggested-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>