automount: do not start expiration timer for TimeoutIdleSec=0

The timer value for automount unit specified with TimeoutIdleSec= is rounded
up to one second if that directive is set to 0.

Fix this by bailing early in automount_enter_runnning() in case no timeout is
requested.

src/core/automount.c

@@ -672,6 +672,9 @@ static int automount_start_expire(Automount *a) {
 
         assert(a);
 
+        if (a->timeout_idle_usec == 0)
+                return 0;
+
         timeout = now(CLOCK_MONOTONIC) + MAX(a->timeout_idle_usec/3, USEC_PER_SEC);
 
         if (a->expire_event_source) {