selftests: mptcp: join: check CURRESTAB counters

selftests: mptcp: join: check CURRESTAB counters

This patch adds a new helper chk_cestab_nr() to check the current
established connections counter MIB_CURRESTAB. Set the newly added
variables cestab_ns1 and cestab_ns2 to indicate how many connections
are expected in ns1 or ns2.

Invoke check_cestab() to check the counter during the connection in
do_transfer() and invoke chk_cestab_nr() to re-check it when the
connection closed. These checks are embedded in add_tests().

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