selftests: mptcp: add csum mib check for mptcp_connect

This patch added the data checksum error mib counters check for the
script mptcp_connect.sh when the data checksum is enabled.

In do_transfer(), got the mib counters twice, before and after running
the mptcp_connect commands. The latter minus the former is the actual
number of the data checksum mib counter.

The output looks like this:

ns1 MPTCP -> ns2 (dead:beef:1::2:10007) MPTCP   (duration    86ms) [ OK ]
ns1 MPTCP -> ns2 (10.0.2.1:10008      ) MPTCP   (duration    66ms) [ FAIL ]
server got 1 data checksum error[s]

Fixes: 94d66ba1 ("selftests: mptcp: enable checksum in mptcp_connect.sh")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/255


Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>