net: Correct the comment of dst_dev_put()

Since commit 8d7017fd ("blackhole_netdev: use blackhole_netdev to
invalidate dst entries"), we use blackhole_netdev to invalidate dst entries
instead of loopback device anymore.
Signed-off-by: NMiaohe Lin <linmiaohe@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dst.c

@@ -144,7 +144,7 @@ static void dst_destroy_rcu(struct rcu_head *head)
 
 /* Operations to mark dst as DEAD and clean up the net device referenced
  * by dst:
- * 1. put the dst under loopback interface and discard all tx/rx packets
+ * 1. put the dst under blackhole interface and discard all tx/rx packets
  *    on this route.
  * 2. release the net_device
  * This function should be called when removing routes from the fib tree