tun: don't require serialization lock on tx

The current tun_net_xmit() implementation don't need any external
lock since it relies on rcu protection for the tun data structure
and on socket queue lock for skb queuing.

This patch set the NETIF_F_LLTX feature bit in the tun device, so
that on xmit, in absence of qdisc, no serialization lock is acquired
by the caller.

The user space can remove the default tun qdisc with:

tc qdisc replace dev <tun device name> root noqueue

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>