selftests: mptcp: diag: check CURRESTAB counters

This patch adds a new helper chk_msk_cestab() to check the current
established connections counter MIB_CURRESTAB in diag.sh. Invoke it
to check the counter during the connection after every chk_msk_inuse().

Signed-off-by: Geliang Tang <geliang.tang@linux.dev>
Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>