selftests: mptcp: add remove addr and subflow test cases

This patch added the remove addr and subflow test cases and two new
functions.

The first function run_remove_tests calls do_transfer with two new
arguments, rm_nr_ns1 and rm_nr_ns2, for the numbers of addresses should be
removed during the transfer process in namespace 1 and namespace 2.

If both these two arguments are 0, we do the join test cases with
"mptcp_connect -j" command. Otherwise, do the remove test cases with
"mptcp_connect -r" command.

The second function chk_rm_nr checks the RM_ADDR related mibs's counters.

The output of the test cases looks like this:

11 remove single subflow           syn[ ok ] - synack[ ok ] - ack[ ok ]
                                   rm [ ok ] - sf    [ ok ]
12 remove multiple subflows        syn[ ok ] - synack[ ok ] - ack[ ok ]
                                   rm [ ok ] - sf    [ ok ]
13 remove single address           syn[ ok ] - synack[ ok ] - ack[ ok ]
                                   add[ ok ] - echo  [ ok ]
                                   rm [ ok ] - sf    [ ok ]
14 remove subflow and signal       syn[ ok ] - synack[ ok ] - ack[ ok ]
                                   add[ ok ] - echo  [ ok ]
                                   rm [ ok ] - sf    [ ok ]
15 remove subflows and signal      syn[ ok ] - synack[ ok ] - ack[ ok ]
                                   add[ ok ] - echo  [ ok ]
                                   rm [ ok ] - sf    [ ok ]

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>