selftests: mptcp: fix error path

pm_nl_check_endpoint() currently calls an not existing helper
to mark the test as failed. Fix the wrong call.

Fixes: 03668c6 ("selftests: mptcp: join: rework detailed report")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Paolo Abeni

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

@@ -661,7 +661,7 @@ pm_nl_check_endpoint()
 	done
 
 	if [ -z "${id}" ]; then
-		test_fail "bad test - missing endpoint id"
+		fail_test "bad test - missing endpoint id"
 		return
 	fi