tun/macvtap: use consume_skb() instead of kfree_skb() when needed

To be more friendly with drop monitor, we should only call kfree_skb() when
the packets were dropped and use consume_skb() in other cases.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>