ptnet: use napi_alloc_skb() rather than netdev_alloc_skb_ip_align()
ptnet: use napi_alloc_skb() rather than netdev_alloc_skb_ip_align()

The new version is optimized to be called by NAPI poll routines.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap