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: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>