automount: lower the idle polling frequency a bit

The autofs kernel idle logic requires us to poll the kernel for
idleness. This is of course suboptimal, but cannot be fixed without
kernel change.

Currently the polling frequency is set to 1/10 of the idle timeout. This
is quite high, as seen in #571. Let's lower this to 1/3.

src/core/automount.c

@@ -672,7 +672,7 @@ static int automount_start_expire(Automount *a) {
 
         assert(a);
 
-        timeout = now(CLOCK_MONOTONIC) + MAX(a->timeout_idle_usec/10, USEC_PER_SEC);
+        timeout = now(CLOCK_MONOTONIC) + MAX(a->timeout_idle_usec/3, USEC_PER_SEC);
 
         if (a->expire_event_source) {
                 r = sd_event_source_set_time(a->expire_event_source, timeout);