block: avoid setting wbt_lat_usec to current value

There's no reason to set wbt min lat and freeze request queue
if current value is the same.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Aleksei Zakharov <zakharov.a.g@yandex.ru>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-sysfs.c

@@ -468,6 +468,9 @@ static ssize_t queue_wb_lat_store(struct request_queue *q, const char *page,
 	else if (val >= 0)
 		val *= 1000ULL;
 
+	if (wbt_get_min_lat(q) == val)
+		return count;
+
 	/*
 	 * Ensure that the queue is idled, in case the latency update
 	 * ends up either enabling or disabling wbt completely. We can't