net: increase skb->users instead of skb_clone()

In dev_queue_xmit_nit(), we have to clone skbs as we need to mangle skbs,
however, we don't need to clone skbs for all the packet_types.

Except for the first packet_type, we increase skb->users instead of
skb_clone().

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>