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>

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -1441,6 +1441,15 @@ chk_join_nr()
 		fi
 	fi
 
+	count=$(mptcp_lib_get_counter ${ns2} "MPTcpExtMPJoinSynAckHMacFailure")
+	if [ -z "$count" ]; then
+		rc=${KSFT_SKIP}
+	elif [ "$count" != "0" ]; then
+		rc=${KSFT_FAIL}
+		print_check "synack HMAC"
+		fail_test "got $count JOIN[s] synack HMAC failure expected 0"
+	fi
+
 	count=$(mptcp_lib_get_counter ${ns1} "MPTcpExtMPJoinAckRx")
 	if [ -z "$count" ]; then
 		rc=${KSFT_SKIP}
@@ -1450,6 +1459,15 @@ chk_join_nr()
 		fail_test "got $count JOIN[s] ack rx expected $ack_nr"
 	fi
 
+	count=$(mptcp_lib_get_counter ${ns1} "MPTcpExtMPJoinAckHMacFailure")
+	if [ -z "$count" ]; then
+		rc=${KSFT_SKIP}
+	elif [ "$count" != "0" ]; then
+		rc=${KSFT_FAIL}
+		print_check "ack HMAC"
+		fail_test "got $count JOIN[s] ack HMAC failure expected 0"
+	fi
+
 	print_results "join Rx" ${rc}
 
 	join_syn_tx="${join_syn_tx:-${syn_nr}}" \