selftests: mptcp: join: correctly check for no RST

The commit mentioned below was more tolerant with the number of RST seen
during a test because in some uncontrollable situations, multiple RST
can be generated.

But it was not taking into account the case where no RST are expected:
this validation was then no longer reporting issues for the 0 RST case
because it is not possible to have less than 0 RST in the counter. This
patch fixes the issue by adding a specific condition.

Fixes: 6bf41020

 ("selftests: mptcp: update and extend fastclose test-cases")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231018-send-net-20231018-v1-1-17ecb002e41d@kernel.org


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