selftests: mptcp: add chk_subflows_total helper

This patch adds a new helper chk_subflows_total(), in it use the newly
added counter mptcpi_subflows_total to get the "correct" amount of
subflows, including the initial one.

To be compatible with old 'ss' or kernel versions not supporting this
counter, get the total subflows by listing TCP connections that are
MPTCP subflows:

    ss -ti state state established state syn-sent state syn-recv |
        grep -c tcp-ulp-mptcp.

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231128-send-net-next-2023107-v4-3-8d6b94150f6b@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>