selftests: mptcp: join: exit after usage()

With an error if it is an unknown option.
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -2077,8 +2077,14 @@ all_tests()
 	fullmesh_tests
 }
 
+# [$1: error message]
 usage()
 {
+	if [ -n "${1}" ]; then
+		echo "${1}"
+		ret=1
+	fi
+
 	echo "mptcp_join usage:"
 	echo "  -f subflows_tests"
 	echo "  -e subflows_error_tests"
@@ -2099,6 +2105,8 @@ usage()
 	echo "  -C enable data checksum"
 	echo "  -i use ip mptcp"
 	echo "  -h help"
+
+	exit ${ret}
 }
 
 sin=$(mktemp)
@@ -2187,9 +2195,12 @@ while getopts 'fesltra64bpkdmchCSi' opt; do
 			;;
 		i)
 			;;
-		h | *)
+		h)
 			usage
 			;;
+		*)
+			usage "Unknown option: -${opt}"
+			;;
 	esac
 done