netdev: Properly clear 'details' when iterating in NETDEV_QOS_FOR_EACH.

The function comment for netdev_queue_dump_next() said that it cleared its
'detail' argument, but it didn't actually do that, which meant that details
could be incorrectly carried along from one queue to the next.

Reported-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>

lib/netdev.c

@@ -1801,8 +1801,9 @@ bool
 netdev_queue_dump_next(struct netdev_queue_dump *dump,
                        unsigned int *queue_id, struct smap *details)
 {
-    const struct netdev *netdev = dump->netdev;
+    smap_clear(details);
 
+    const struct netdev *netdev = dump->netdev;
     if (dump->error) {
         return false;
     }