extent scan: extra check to make sure no Tasks are started when throttled

Previously `scan()` would run the extent scan loop once, and enqueue one
extent, before checking for throttling.  Do an extra check before that,
and bail out so that zero extents are enqueued when throttled.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-roots.cc

@@ -899,6 +899,9 @@ BeesScanModeExtent::scan()
 {
 	BEESTRACE("bsm scan");
 
+	// Do nothing if we are throttled
+	if (should_throttle()) return;
+
 	unique_lock<mutex> lock(m_mutex);
 	const auto size_tiers_copy = m_size_tiers;
 	lock.unlock();