tuntap: restore default qdisc

After commit f84bb1ea ("net: fix IFF_NO_QUEUE for drivers using
alloc_netdev"), default qdisc was changed to noqueue because
tuntap does not set tx_queue_len during .setup(). This patch restores
default qdisc by setting tx_queue_len in tun_setup().

Fixes: f84bb1ea ("net: fix IFF_NO_QUEUE for drivers using alloc_netdev")
Cc: Phil Sutter <phil@nwl.cc>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>