selftests: mptcp: validate MPJoin HMacFailure counters

The parent commit fixes an issue around these counters where one of them
-- MPJoinAckHMacFailure -- was wrongly incremented in some cases.

This makes sure the counter is always 0. It should be incremented only
in case of corruption, or a wrong implementation, which should not be
the case in these selftests.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250407-net-mptcp-hmac-failure-mib-v1-2-3c9ecd0a3a50@kernel.org


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