dm cache: fix incorrect 'idle_time' reset in IO tracker

Some bios have no payload (eg, a FLUSH), don't reset the idle_time when
these come in.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-target.c

@@ -94,6 +94,9 @@ static void iot_io_begin(struct io_tracker *iot, sector_t len)
 
 static void __iot_io_end(struct io_tracker *iot, sector_t len)
 {
+	if (!len)
+		return;
+
 	iot->in_flight -= len;
 	if (!iot->in_flight)
 		iot->idle_time = jiffies;